Students are set to move out of marquee classrooms after almost three years of being taught in tents.
A two-storey L-shaped building with 24 temporary classrooms, seminar rooms, offices and toilets are being built at Clevedon School.
Pupils have been taught in the marquees since 2023, when the discovery of high alumina cement concrete (HACC) in the site's large beams caused 22 classrooms to shut.
Jim Smith, the secondary school's head teacher, said: "The new facilities will be a much needed boost to the school community that has coped so well in the emergency accommodation, which in the immediate discovery of HACC, meant we were able to keep all students in school."
HACC is a weakness in concrete that can lead to sudden and catastrophic collapse. It was found, along with reinforced autoclaved aerated concrete, in scores of school across England in 2022 to 2023.
Clevedon School's enabling works, which include access, safety fencing and welfare for the temporary learning spaces, began on Monday.
It is hoped the new classrooms will open in time for the next academic year, which begins in September.
