#104 maybe at your uni you can't even get into a building without id but that isn't the case everywhere! #106 usually universities are very large compared to high schools, so staff wouldn't necessarily know the op, unless it is a very small campus or something. and for what sounds like a very important exam, you can't just explain the situation - you might get away with that on a small assignment or something (depending where you go, of course), but for something like a final exam you would normally need documentation of some sort, otherwise anyone could claim that something like this happened to get out of an exam they weren't prepared for.

When you go to huge colleges and uni's often the tests are taken in separate building, and you have to show identity to show you are a student.