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.
- Mum About Town with Angellica Bell: 'Thank you to all my readers over the past three years'
- Most read stories across Kingston of 2014
- Police deliver 200 presents to cared-for children in Christmas appeal
- Safety fears over roof garden proposal for 300-bed student flats scheme
- Police close case on Kingston town centre 'attack'