Technically a school can expel you for anything they want to (unless its otherwise exempted by federal law--ie race, religion, etc......assuming you are at a public school, private schools are different but since pretty much all of them accept federal funds/loans, this would also apply to them.) But I can't imagine a school would actually expel you for being late twice to class (even a rotation probably wouldn't fail you for that.....but more than twice would be pushing it.)
That said, why are you taking your Adderal so late in the day? If you know it affects your sleep, you need to take it earlier. It sounds like you are taking it later, so you can stay up later to study (or party?) This is a bad idea, take your adderal at the same time every day. And then get to class on time, everyone has a sleepless night from time to time for a variety of reasons (storms, crying baby, emotional state, etc.).....your workplace won't care and will fire you for this. So use school for practice, to figure out what you need to do to be able to be on time.