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.”
Jamal Elmi said he was walking past the scene earlier in the day.
He said: “I had walked past before, earlier when the kids were coming out as usual and it was peaceful.
“I came back an hour later and it was like a warzone.
“There were police and ambulances, fire engines – everything was all sealed off.
“Then I realised it was an acid attack and a teacher was involved.
“It’s actually lucky that more people weren’t hurt if it was an hour earlier it was so busy.”
A Met police spokesperson said: “Police on patrol were flagged down at 16:42hrs on Monday, 30 September by staff at a school in Alfred Road W2 after a substance was allegedly thrown at pupils and a staff member.
“Officers responded with paramedics and the London Fire Brigade and found three people injured.
“A 16-year-old boy, whose injuries are not life-threatening or life-changing. He remains in hospital.
“A 27-year-old woman – whose condition is not life-threatening or life-changing has been discharged from hospital. A 14-year-old girl, whose injuries may potentially be life-changing. She remains in hospital.”
Scotland Yard said tests of the substance are ongoing, adding: “At this stage officers believe it to be acidic. Urgent enquiries are underway to identify and arrest those responsible.”
Headteacher Numera Anwar said: “One pupil, a teacher and a member of the public have been injured in an incident outside the school building on the main road involving an acid-like corrosive substance.
“This was a distressing incident which took place after school hours when students and staff were leaving the school site. We are offering support to families and relatives of all involved.
“This is a difficult time for our community and I took the decision to close the school today to allow for a thorough investigation and to ensure the school can resume normally from tomorrow with all safety measures in place, such as additional patrols from local police officers.”
She said that she plans to reopen the school tomorrow.
Leader of Westminster City Council, Cllr Adam Hug, described the attack as “truly shocking”.
“The council has been in touch with Westminster Academy and we will provide educational psychologists and any other support to help children through what is a traumatic ordeal,” he said.
“Our thoughts are with all those who have been affected.
“Clearly this attack will be unsettling for the whole Academy and the wider community, and we will be working with police to provide reassurance and whatever support we can. We would urge anyone with information about what has happened to contact the police.”
Anyone who witnessed the incident, or who has information that could help police, should call 101 ref CAD 4987/30 Sept.
To remain 100% anonymous contact the independent charity Crimestoppers on 0800 555 111.