T.O.
It's normal and I wouldn't worry. Just remind him to go potty before recess, after lunch etc. It's probably a lack of control, and he's having so much fun he just doesn't want to stop and take the time to go potty.
Also, make sure the teacher is giving every opportunity to go. My daughter at 5 had an accident and it was because there was only 1 working bathroom, 32 kids, and she was in line waiting, but just couldn't keep holding it. Of course, that was California and the schools suck there, so it probably isn't the case for your child.