Emergency services were alerted to the blaze at Musselburgh Grammar School on Thursday afternoon.
According to reports, pupils and staff were evacuated from the building as the fire was extinguished.
There were no injuries as a result of the fire.
Police said they are treating the incident as ‘wilful’.
A Police Scotland spokesperson said: “Around 1.20pm on Thursday, 26 May, 2022, emergency services were called to a report of a fire at Musselburgh Grammar School, Inveresk Road, Musselburgh.
“The fire is being treated as wilful and officers are following a positive line of enquiry.”