Four fire engines and around 25 firefighters were called to a fire at a primary school on Cookhill Road in Abbey Wood.
A single-storey timber mobile classroom within the grounds of the school was destroyed by the fire. Part of the external fascia of the main school building was also damaged. There were no reports of any injuries.
A London Fire Brigade spokesperson said: "This fire was discovered by a member of staff who saw the fire in the classroom.
"They attempted to tackle the blaze themselves and then called us.
"You should never try to tackle a fire yourself. Leave it to the professionals. As soon as you become aware of a fire, get out, stay out, close the door behind you and call 999.”
The Brigade's 999 Control Officers received nine calls to the fire.
The Brigade was called at 1208 and the fire was under control by 1351. Fire crews from Plumstead, Erith, Bexley and East Greenwich fire stations attended the scene.
The cause of the fire is under investigation.