Tolworth Girls' School is closed today for all students following a fire.
The 100sqm single-storey hut has been completely damaged by the flames, the brigade said.
It is the second incident at the school this week, after fire broke out in a classroom the early hours of Monday morning.
Headteacher Siobhan Lowe said a caretaker raised the alarm.
She said: "No one was injured and no lives were ever at risk.
"[Staff] decided to close the school to allow the fire brigade to check for smoke damage and also to make sure that the building is safe.
"The fire crews have been stunning. The police have been amazing as well - they have kept us informed throughout."
The school's English block sustained some smoke damage, Mrs Lowe said.
The incinerated classroom hut was used to teach health and social care, psychology, and child development, she added.
Firefighters were called at about 5.30am, with New Malden and Surbiton crews arriving first and later Kingston for extra assistance.
Mitcham fire station manager Jeff Lisle said: "On arrival it was obviously a serious fire.
"It was in a single storey prefabricated building.
"We managed to provide covering jets for surrounding buildings to prevent the fire spreading.
"We had difficulties extinguishing the fire due to the construction.
"It is very difficult to get into certain areas."
No other buildings were damaged.
An investigation into the cause of the blaze is underway, Mr Lisle added.
- Police link three 'targeted' violent attacks on boys
- Case for Heathrow expansion 'stronger since Brexit' says airports review chief
- Body found in search for man missing from Epsom
- UPDATED: Dead body on tracks causes train delays
- South Londoners making most of new hopper bus fare - especially these two routes