Does he only wet himself when he is at school, or also at other times? How often does it happen? Does he wet the bed? There could be any number of reasons and you didn’t give very much information. If he has other bladder control problems, he should be checked by a doctor, but if it only happens at school then there is probably something going on at school that causes him to wet his pants. He might find his class work too stressful, he may have a teacher that won’t let him be excused when he needs to go, or other kids may be picking on him or bullying him. Try talking to him and maybe also to his teacher to see if you can find out what the problem is.