Stephen Hammond, the local MP for Wimbledon, has confirmed the school was having its last day of term.
Speaking at the scene, Mr Hammond said: "It was very clear, very quickly, that it was a major incident and the local police declared it just that.
"I think you will have seen by the sheer scale of the response from the paramedics, air ambulance, fire brigade and the police what an important and serious incident this is.
"And what a sad incident - it is tragically on the last day of term, for this young girl to have lost her life.
"This is a well-liked, well-respected, well-known school.
"Most of the children who go here will be Wimbledon families, and this will cause shock across the whole of the local community."