Four pupils have been arrested on suspicion of arson by police officers investigating a series of fires at Harrow High School earlier this week.
Two of the group of three girls and one boy, all 14, were excluded from the secondary in Gayton Road, Harrow, following the arrests.
Four small fires - two in the main building and two in the sports hall - broke out in suspicious circumstances on Tuesday (Sept 23) at 2.30pm, forcing the whole school to be evacuated.
Staff managed to extinguish the flames themselves, although Harrow police and firefighters attended as a precaution.
Vivien Swaida, headteacher of Harrow High School, a specialist sports college, said: "We are co-operating fully with the police after this deeply irresponsible act and we are grateful that no-one was hurt.
"It is extremely upsetting that anyone would want to damage school property in this way. However, the school has already taken swift action on this matter and we can confirm that two pupils have been excluded from the school.
"The school is operating normally and we held our parents' evening yesterday as planned. This is an outstanding school where the vast majority of students have a clear sense of community which they demonstrated clearly yesterday."