I've got a bag, so it's a bit hard. I'm in a new school now and don't know many people, but in my last school, I'd let my close friends know I had Crohns and they were chill, and my best friend also has IBS so he gets it. But everyone else just treats me for me, because no one else knows, but usually people won't judge you for it.
Also carrying supplies/medicine around in a small compartment works, and the school system and nurse knowing helps A LOT, the 504 plan (in the US) is a good benefit too.