A 35-year-old man has been arrested on suspicion of causing grievous bodily harm after a suspected acid attack outside a school in west London.
The Metropolitan Police said the man was arrested on Thursday morning and remains in custody after the incident outside Westminster Academy on Monday.
A 14-year-old girl who sustained injuries which are "being treated as potentially life-changing" has returned to hospital having been briefly discharged by doctors, police said.
A 16-year-old boy has been discharged by the hospital, as police said his injuries were "not as severe as the girl's".
Detective Superintendent Scott Ware, investigating the incident, said: "The investigation team has been working tirelessly to piece together the events in which two young victims, aged 16 and 14, were approached outside the school by a lone man who threw a substance at them before fleeing."