Most large colleges/universities... Mine wasn't even that big and I almost always had to bring ID to exams.

Wow. I guess I'm lucky. I go to the largest university in my state, and I just had to know my student number to put it on the sheet.

In large classes (100+ students) you need an ID to hand in your test at my university. If you sit in the front row every day or talk to the instructors a lot, they recognize you and you don't need to show it.

With a large enough student population, it's easily possible that the profs wouldn't know everyone (especially if they're from a different faculty or something). And even some high schools require photo ID for students to be allowed to write their final exams.

i live in australia and i know our universities are a bit different to american ones and probably those in other countries too, but at my uni (which is fairly small compared to most in australia), we are ALWAYS required to take id to any exam because the staff don't usually know all the students (some may not even know the students in their own classes), and major exams (e.g. finals) are conducted by separate exam supervisors, not by the teaching staff, so obviously they wouldn't know anybody. come to think of it, even in my HSC (final high-school exams) students had to have photo id, because again the tests were conducted by official examiners, not by our teachers... what happened to you sucks but surely you will be able to sit a make-up test. i have had to do this several times for medical reasons and they have always been very helpful about it, so if the staff at your uni are anything like this then it will probably work out fine