Fire fighters were called to a blaze at Stanley Park High School this morning.
Four fire engines and 25 fire fighters were called to the school in Damson Way, Carshalton, at 9.55am.
It is understood that a photocopier caught fire due to an electrical fault.
The fire, which was confined to a small part of the room, had been mostly extinguished by the school's sprinkler system before the fire brigade arrived.
About 1,100 pupils were safely evacuated and only part of the room has been damaged.
There are no reports of any injuries but the school will be closed until Thursday because of damage caused by the water used to put it out.
The school issued a statement on its website which said: "All students and staff were safely evacuated, with the behaviour of our students being highly commended.
"Although the fire was put out quickly, water from the fire engine and our sprinkler system has resulted in areas of the school, particularly Trade, being unusable.
"Following advice from the local authority, we have asked for special closure. This has been agreed by Ms Pascoe, chair of governors."
Students were allowed to go home early provided they assured the school it was safe for them to do so.
Stanley Park will reopen on Thursday, June 27, at the usual time.