The school's Head of History, Susan Casey, says it is a remarkable find.
She is hoping money can be raised to conserve the shelter and turning it into a teaching facility.
More top news
Football blog on Sunderland's game against Stoke, by Martyn McFadden
Hundreds of people have joined a walk in memory of a teenager from Newcastle who died from suspected meningitis.
Emma Jesson brings you Saturday night's forecast