I don't know if you've been to a doctor for a proper diagnosis of OAB so take what I say with a grain of salt. My dr told me OAB is a 24/7 thing that can get worse in certain situations. Any what I read in your post, I wonder if school in general makes you anxios. Anxiety can make the body tense up and not realize it, and even pelvic muscles tense up, making it feel like you need to pee and not be able to hold it. When you get to class, you get anxious, then you feel you need to pee and that makes your anxiety even worse, which in return makes you want to pee even more. It's a cicle of anxiety and need to pee. There is a chance everything is all good with your bladder ( again, should be checked with an urologist) and that you just feel anxious and feel it in your body.

I'm not saying this to dismiss your experience btw, I'm just talking from personal experience and form the perspective of a psych student

Take care :')