Pupils from St Mary’s Catholic Technology College were sent home yesterday after the electricity supply to the Royal Avenue school was severed.
Headteacher Kathy McNicholas, said: “We had an unexplained loss of power at school on Monday morning, and once we realised it wasn’t a fleeting power cut, we began our procedure of contacting parents so pupils could safely return home.
“It was some time after lunch before full power was restored and the few students who were still at school were able to continue their day.
“We are still looking into what might have caused the power cut but we expect school to continue as normal today.”
The school was ravaged by fire last year and is currently being partially rebuilt.
Some students have been housed in portable classrooms while building work is carried out on the site.