Police are investigating the cause of a fire at Pardes House Grammar School in Finchley on Sunday afternoon.
Firefighters were called to the Hendon Lane premises after staff smelled smoke.
The blaze caused around £1,000 worth of damage in a store room, setting alight old desks and chairs.
Some boys were seen running away from the school. It is not known whether they were Pardes House pupils. CCTV footage is being studied in an attempt to identify the group. Barnet Police say there is nothing to suggest the fire was motivated by antisemitism.