A 14-year-old schoolgirl suffered potentially life-changing injuries and three others were injured in a suspected acid attack.
Police have launched a manhunt after three people were injured in the attack outside Westminster Academy in central London at 4.45pm on Monday.
Two police officers who were flagged down were also taken to hospital after saying they felt unwell.
A Met spokesman 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.”
Two officers were taken to hospital as a precaution after they reported feeling unwell.
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.
“A number of crime scenes are in place.”
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.