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
The Queen has paid a visit to Royal Manchester Children's hospital where she met young people injured in Monday night's terror attack.
Scientists from Newcastle University believe they have found an easy way to validate the authenticity of any paper document.
Friends of a York student who lost her parents in the Manchester terror attack have launched a crowdfunding campaign to support her family.