Fire investigators believe a blaze at Southend High School for Girls could be arson.
Crews from Southend and Shoebury were called to the school, in Southchurch Boulevard, on Monday morning.
The blaze started in an art room, forcing the evacuation of the school.
Officers say they have investigated every possible cause of for the fire and arson is the only remaining possibility.
Lessons were cancelled for the two days.
The school will reopen on Wednesday.
Nearly one-thousand pupils attend the school but no one was hurt.