A 14-year-old schoolgirl suffered potentially life-changing injuries and two others were hurt in a suspected acid attack.
Police have launched a manhunt after the people, including a teacher, were wounded in the attack outside Westminster Academy in central London at 4.45pm on Monday.
Two police officers, who were among the first to the scene, were also taken to hospital after saying they felt unwell.
A 27-year-old woman, believed to be a teacher who helped the victim, has since been discharged, while a schoolboy, 16, whose injuries are not life-threatening or life-changing remains in hospital.
Police are hunting a suspect described as a tall, slim, Black male, wearing dark clothing with his face obscured, possibly by a mask or balaclava. He fled the scene on an e-scooter.
The academy was shut on Tuesday morning with police patrolling outside. Staff were seen through the windows conducting meetings inside.
A pharmacist who works nearby said: “We saw police rushing to the scene. Then lots of people running away from where it happened. They were all gathering by the school entrance after. It is so sad. We thought it was a stabbing then we heard it was an acid attack.
“You need to put cold water on as soon as possible when that happens. It is horrific.”
Another parent living in a home overlooking the scene said: “I saw so many ambulances, fire brigade, everything. I go to church and pray for my kids every day even when they go to school.”
DCI Dean Purvis, who is leading the investigation, said: “Officers were on routine patrol when they were flagged down to this terrible incident.
“From our enquiries so far we believe the two young victims were on Alfred Road when they were approached by a lone suspect who threw a substance at them before fleeing down Harrow Road.
“A member of staff bravely and selflessly rushed to provide first aid to the two victims, sustaining injuries herself in the process. Fortunately she has now left hospital.
“The two teenage victims – a girl and a boy – remain in hospital. For the 14-year-old girl, we might not know for some time how severe her injuries are but at this time they are being treated as life-changing.
“I know how incredibly concerning this incident is to parents and pupils and the wider Westminster community. We are in close contact with the school and local authority, and I can assure people that a thorough investigation is well underway, and that my officers will not stop until the suspect is found and arrested.
“If you have information that can help us, it is imperative that you tell us immediately.
“The area at the time was busy with people and cars. I am particularly interested in hearing from anyone who might have captured dash cam footage that can help us establish what happened.”
DCI Purvis added: “I would like to acknowledge the incredible bravery of all the emergency services who responded, including our London Ambulance Service and London Fire Brigade colleagues.
“Two officers were taken to hospital as a precaution after they reported feeling unwell.
“I am happy to update that they are both well and have left hospital, and we will continue to look after their welfare.”
