S.R.
We had the same problem when my 4-yr-old started Pre-K. He's been in school this whole school year and has done fine. He's learned that he can wipe himself, unless it's a very messy one. In that case, he asks for help. The teachers don't really like to do this I think, but they will if they have to. Also, we get up early enough to let him try to go before we leave for school. Because I've made this a habit, I think his body is sort of trained to go in the mornings. He usually does his business in the mornings, that way I can help him wipe and he doesn't have to worry about it at school. Occasionally he can't go in the am and does have to go later at school, but not very often.
Good luck! These are the parts of mommyhood no one tells you before you have kids, huh? Lol!