Workington Academy will fully reopen tomorrow.
The school was hit by water and heating problems and was closed to all pupils on Wednesday and opened today for year 11, 12 and 13 pupils.
The academy's interim head, Colette Macklin, said staff noticed damp spots and condensation on the walls so a team of engineers was called out.
They have worked through the night to try and find a cause to the problem and investigations are continuing again today.
"We think there may be some historical leakage underneath the building, but we can't isolate anything as specific at the moment," said Mrs Macklin.
"The water and heating has been turned off as a precaution and staff were in the academy.."
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