Two boys, aged 12 and 13, are in a serious condition in hospital after Tuesday's attack in Brent.
Police launched a search for the suspect, and officers later detained a 13-year-old boy on suspicion of attempted murder.
Follow the latest updates on attack
The investigation is being led by counter terror police, although a terrorist incident has not been declared.
Here's everything we know so far.
What happened at the school?
Police were called to Kingsbury High School around 12.40pm after a report that a 13-year-old had been stabbed.
Officers then became aware that a second boy, aged 12, had also been stabbed.
The Metropolitan Police said officers began a search for the suspect - identified as a teenage boy - after he left the scene.
Shortly after 6pm, Detective Chief Superintendent Luke Williams confirmed in an update that a 13-year-old boy had been arrested on suspicion of attempted murder.
A weapon was also recovered.
Who are the victims?
The two stabbing victims were taken to hospital in a "serious" condition, with one taken to a major trauma centre as a priority.
The identity of the boys, who are aged 12 and 13, remains unknown.
The London Ambulance Service (LSA) was called to the school at 12.41pm.
"We sent resources to the scene including ambulance crews, incident response officers, an advanced paramedic, a paramedic from our tactical response unit and a clinical team manager," the LSA said.
"We also dispatched a trauma team in a car from London's Air Ambulance.
"We treated two patients in total. We took one patient to hospital and the other as a priority to a major trauma centre."
Who is the suspect?
Police have arrested a 13-year-old boy on suspicion of attempted murder.
The boy - whose identity is not known - remains in custody for questioning by police.
What have police said?
DCS Williams said police are "keeping an open mind as to any motivation behind the attack".
He said the investigation was being led by counter terrorism police, though a terror incident has not been declared.
He said counter terror officers are "working closely with our local officers in response to this incident".
"The investigation has not currently been declared a terrorist incident, and the investigation team will now be working at pace to gather any evidence and to establish the full circumstances of what's happened as quickly as possible."
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What has the school said?
Kingsbury High School head teacher Alex Thomas told parents that Tuesday's events marked a "deeply traumatic event for the whole school community".
In a letter to parents, Mr Thomas said pupils at the lower school had been kept on the playground at the request of the police while they began their investigations.
He added that the lower school will be shut on Wednesday, but the upper school will remain open for students in years 10 to 13.
The school released a statement at about 4pm saying that the "serious incident" was under control and it was not possible to enter or leave the school site.
As William walked off a football pitch on the outskirts of Riyadh, where he is on an official visit, a reporter asked the prince: "Sir, to what extent do you think the Royal Family has done enough around the Andrew and Epstein issue?"
The palace is trying to keep this trip to Saudi on track, with no cancellations or obvious adjustments to engagements.
They want to remind us that William is used to navigating difficult times and takes his role seriously. If nothing else, they hope the heir and his family show the monarchy has got a positive future, at a time when they really need it, as they continue to compete against the seemingly uncontrollable noise around Andrew and Epstein.
It's clear from the statements released on Monday that something has shifted over the past few days.
William and Kate publicly addressed the Epstein scandal for the first time, with Kensington Palace saying they were "deeply concerned" at the "continued revelations" and that their thoughts "remain focused on the victims".
Meanwhile, Buckingham Palace said the King had "profound concern" over claims about Andrew and it would support police if contacted.
Silence was not an option, with the questions about what information Andrew was sharing during his time as trade envoy going right to the heart of the Royal Family's public purpose and what they're meant to be about.
Andrew has denied any wrongdoing in relation to Epstein.
Neither William nor the King have spoken about it on camera - not really a surprise - and William wasn't going to do it here in Saudi.
But it doesn't help quell the voices of those who say they want to hear more from them - visible proof, you might say, of the "concerns" they have written about.
William will also no doubt have seen the calls for the King to encourage Andrew to voluntarily speak to the police and authorities in America. We know that even on official trips, he monitors the news back home.
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But here in Saudi there was a new global audience for him to think about - one that wasn't going to ask him about Epstein.
William and Kate's social media accounts feature a stream of photos of him meeting and greeting, and especially focusing on the time he spent with children and women.
For the palace, it was a day meant to be about important future relationships as headlines in the UK continue to dwell on the past.
Lucy Harrison, a fashion buyer from Warrington, Cheshire, was staying with her father, Kris Harrison, and his family at their home in Prosper, Texas, in January last year.
Ms Harrison's boyfriend, Sam Littler, who made the trip with her, told Cheshire Coroner's Court she would often become upset with her father when he spoke about his gun ownership.
He said that on the morning of 10 January - when the couple were due to fly home - Ms Harrison and her father argued about Donald Trump.
"Kris and Lucy ended up having quite a big argument which led to Lucy running upstairs and being upset," Mr Littler said.
He said Ms Harrison asked her father how he'd feel if she was sexually assaulted.
Her father replied that he had two other daughters who lived with him so it would not upset him that much, the inquest heard.
Then, Mr Littler said, about half an hour before they were due to leave for the airport, Mr Harrison took his daughter by the hand and led her into his ground-floor bedroom.
He said he heard a loud bang some 15 seconds later, before Mr Harrison started screaming for his wife, Heather.
Mr Littler said: "I remember running into the room and Lucy was lying on the floor near the entrance to the bathroom and Kris was just screaming, just sort of nonsense."
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Ms Harrison, who worked for fashion brand Boohoo, was described as a "real force of life" by her mother, Jane Coates.
She said: "She cared. She was passionate about things. She loved to have debates about things that meant a lot to her."
The inquest heard a grand jury in the US determined there was insufficient evidence to charge anyone in connection with Ms Harrison's death.
It was also told that Mr Harrison had previously been to rehab for alcohol addiction.
He did not attend the hearing, but his representative, Ana Samuel, said it was "more akin to a criminal investigation than a fact-finding inquiry".
In a statement issued by his solicitors, Mr Harrison said: "I fully accept the consequences of my actions, and there isn't a day I don't feel the weight of that loss - a weight I will carry for the rest of my life, and I know that nothing I say can ease the heartbreak this tragedy has caused.
"I cannot undo what happened, but I can honour Lucy by being the best father I can be to her sisters and by carrying her memory forward in everything we do.
"I am deeply sorry for the pain others feel from this tragedy. Lucy's spirit - her warmth, her humour, her kindness - will live on in all of us who loved her."
The inquest is expected to conclude on Tuesday.
Laegreid finished in third place in the biathlon, which combines cross-country skiing and rifle shooting, and then broke down in tears at the Winter Olympics on Tuesday.
The Norwegian told fans he already "had a gold medal" up until last week - but squandered it by being unfaithful.
Speaking to Norwegian state broadcaster NRK in his native language, just moments after finishing his biathlon, he said: "There's someone I wanted to share it with who might not be watching today.
"Six months ago, I met the love of my life - the most beautiful and kindest person in the world.
"Three months ago, I made my biggest mistake and cheated on her. I told her about it a week ago. It's been the worst week of my life."
The 28-year-old then issued a heartfelt plea but stopped short of naming the woman.
"I had a gold medal in my life, and there are probably many who look at me with different eyes, but I only have eyes for her," he added. "Sport has taken a slightly different place [in my life] the last few days. I wish I could share it with her."
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Laegreid later said he might not have been in the right state of mind to give the interview after the race, which came in the wake of the death of his teammate, Sivert Guttorm Bakken, in December last year.
"Maybe it was really selfish of me to give that interview," he said a few hours later. "So yeah, I don't know. I was, I'm a bit, I don't know... I'm not really here mentally.
"So yeah, we will see what happens."
Laegreid came third in the men's Olympic 20km individual biathlon race in northern Italy. His fellow Norwegian Johan-Olav Botn took gold while France's Eric Perrot claimed the silver.
The race came as US skier Lindsey Vonn confirmed she will need "multiple surgeries" following her crash in the women's downhill skiing final on Sunday.
Sir Richard Moore, who left his post last September, told Sky News' lead world news presenter Yalda Hakim he had not been part of the process, amid more revelations about the ex-Labour peer's relationship with paedophile Jeffrey Epstein.
Sir Keir Starmer last week apologised to the public and to Epstein's victims for believing what he called "Mandelson's lies".
The prime minister insisted he was misled over the extent of Lord Mandelson and Epstein's links, saying he had "no reason" to believe the peer was allegedly lying when he said he barely knew Epstein before being made ambassador.
Sir Keir criticised the vetting process that Lord Mandelson went through. The Epstein files appear to show regular contact between the former Labour grandee and Epstein after the US financier's sex offence conviction in 2008.
Speaking about background checks more broadly, former spy chief Sir Richard said: "Undoubtedly there would have been some background checks and then there's an interview process, where you rely to a degree.
"You've got the background checks, of course, to set it against, but you rely on people in those interview processes being very honest."
He added: "I've been through that many, many times. They can sometimes be uncomfortable conversations because you're being asked about very private elements of your lifestyle or about your financial situation, and you are required to answer those questions truthfully.
"And the system depends on the balance between truthful answers in the interview process and the checks on whether what's been said is truthful."
Read more: What do the Epstein files say about Mandelson?
Sir Richard said he was not aware of any intelligence relating to the peer's appointment as US ambassador, but even if he was, he would not be able to share it.
He added: "I'll say to you, I'm not aware of anything, but I repeat what I said, you have to understand the sensitivities of this, this stuff does not get circulated around the building. So the answer is, I have no idea."
He also called out what he described as the "rancid misogyny and appalling systematic abuse of women and girls" exposed in the Epstein files.
Lord Mandelson was appointed ambassador in December 2024 and was sacked by Sir Keir in September 2025 over the peer's ties to Epstein.
It comes as government figures prepare to hand over huge amounts of material to Parliament's security watchdog after they came under pressure to reveal what was known about the peer's friendship with Epstein when he was picked for the Washington job.
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Meanwhile, the Metropolitan Police has launched an investigation following accusations that Lord Mandelson passed market-sensitive information to Epstein in 2009.