A series of mobiles, built to support a rise in pupil numbers at Hazelwood Integrated College in Newtownabbey, are padlocked.
Hundreds of 6th-formers had to be sent home on the first day of term.
Since then, the school has been teaching its pupils in every available space – including the principal’s office, dining hall and storage rooms.
The row centres on derelict and unoccupied land owned by Belfast Education and Library Board (BELB).
Mobiles, which were built to house the school’s maths department and a new common room, were erected in the summer on the former site of Cedar Lodge special school, which is adjacent to Hazelwood.
The BELB says the buildings appeared on its property without appropriate approvals and posed a health and safety risk.
Hazelwood principal Kathleen Gormley says staff, students and parents are devastated.
Because of the school’s excellent GCSE results this year, it welcomed back its largest ever 6th form with almost 200 students at post-16.
“This new accommodation comprises of our new 6th form centre and students and parents are extremely unhappy,” Ms Gormley said.
“We hope an interim solution can be put in place immediately in the interests of all our pupils.”
The BELB received a written request from Hazelwood in May requesting a lease for the site.
It was reinforced that the acquisition of the lease needed to be compliant with the Board, Department of Education, Belfast City Council Building Control, and Department of Environment approvals.
The BELB said it was inappropriate to commence any work in the interim, as the site remained board property, adding it was made clear that the lease would not be achievable by September 2013.
“Further to a recent visit to the Belfast Board site at Cedar Lodge, it became clear that a building had been erected, without the appropriate approvals,” a spokesman said.
“As the Board still owns the property, any use of the unapproved building may place a health and safety risk to pupils, staff and the general public. As a responsible organisation, the Board has repeatedly secured the building in recent days.”