Weeeeeell now.
As a teacher, LadyB, I have a problem with your blanket statement of "teachers can be mean to kids". I have never in 15 years of teaching seen or heard of a teacher being mean to kids just for the fun of it. Strict or stern, yes - but mean, no.
Mail carriers can be mean to kids, soccer coaches can be mean to kids, ministers can be mean to kids, parents can be mean to kids.... see my point?
MANY people don't understand/underestimate the severity of Crohn's disease. Family members even think people are faking it sometimes.
Andy, hi and welcome!
I don't really have pains, so I'm not sure if I can help, but if you knew what the pain was from - that might help you. Is it from gas? A stricture? Can your doctor pinpoint it? I would think it would be easier to sit all the way in the back so if you have to stand or stretch or lay your head down you won't be the center of attention. This sounds weird, but the few pains I do have are usually on the toilet. Sometimes I did slow deep breathing (like when ladies are having a baby) to help with that. I do in through nose, out through mouth. If you could do that quietly, that might help. Maybe just telling each teacher "I've been in a lot of pain lately, if I appear to be rude, that's not my intention" would make you feel better about acting differently in class.
Good luck, feel better.