The girl remains in hospital with potentially life-changing injuries.
A 16-year-old boy remains in hospital with non life-changing injuries, and the staff member, a 27-year-old woman, has been discharged.
Westminster Academy said the injured boy was a member of the public and not a pupil.
The school was closed on Tuesday, with lessons taking place online and many staff working from home.
The Met believes the two teens were approached on Alfred Road by a male “who threw a substance at them before fleeing down Harrow Road”.
The force described the suspect as a tall and slim black male wearing dark clothing and riding an e-scooter.
