Pupils at Cowbridge comprehensive in the Vale of Glamorgan are now being taught in the main building - including in the library and canteen - while council officials hold urgent talks to try to find a solution.
Martin Blamey, chairman of governors, has written to parents warning that children may be sent home if further structural problems are found.
The 1,200-pupil comprehensive, which last year came third in the GCSE league table for state schools in Wales, with 81 per cent of pupils getting five A* to C grades, has long had problems with its buildings.
It inherited buildings on three sites when it was formed out of three schools in 1974. Some of its portable classrooms are more than 30 years old.