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
A look ahead at the forecast for the Tyne Tees region.
Tributes have been paid to a man who died after collapsing and falling into the river in Durham at the weekend.
Stephen Miller's twentieth senior international season got underway last week at the Fazza International Athletics Competition in Dubai.