Fire crews used water from a college swimming pool to dampen down a blaze at a campus accommodation block.
Seven fire crews put out the blaze which severely damaged the boarding house at Atlantic College, Vale of Glamorgan. No-one was injured.
The college, which has 350 students aged 16-18, said any affected students of 50 based at the building will be accommodated within St Donat's Castle.
South Wales fire service said it received an alert at 1226 BST.
Four crews remain at the college while the extent of the blaze, which hit the ground floor, first floor and roof of the block, is assessed.
In a statement, the college said the blaze broke out at around midday in Sunley boarding house and the alert was raised by an automatic fire alarm system.
The building was evacuated of staff and students, no-one was injured and everyone was accounted for, the college said.
Principal Paul Motte said: "Despite this being a major incident of a very serious nature it is reassuring to know that all members of the college community acted responsibly and all are safe and well.
"We are now working to minimise the impact of the incident on our students' welfare and education."
The college recruits its students from some 80 countries and is housed at St Donat's Castle, near Llantwit Major, where it opened in 1962.
The college was involved in the development of the International Baccalaureate diploma and was one of the first in the world to offer the diploma.
The castle, which dates from the 12th Century, was once owned by the American newspaper mogul William Randolph Hearst.