You give a lid to a person (two of them, in fact), then they put it on a cross when you graduate. It's a symbolic ceremony, putting a lid over a cross, but it's just what you do when you graduate around these parts.

Uh oh, looks like you forgot your lid. They decide not to let you graduate. So two (そつ) more years now until you can graduate, it seems. So two more years?! Just for forgetting a lid? Nonsense!