I haven't had a chance to read the other advice yet, so I'm sorry if I am repeating what has already been said...
Whatever adult is in charge of him at any given time needs to constantly remind him to go. I know his teacher is "busy" with a whole class full of kids, but it is her responsibility when she is with him to be PROACTIVE. She should set up some kind of system with him (like a timer) where he HAS to stop what he is doing and go pee. If she is too inexperienced to know what to do, speak with the center's director. There is no reason for him to continue to have this issue at school-- they KNOW he has not been stopping what he is doing to pee on his own, so they have the responsibility to teach him. A timer is the best way.
Follow through with the same thing at home. Buy a small bakery timer-- the cheap kind that you twist and it rings-- set it every 20 minutes when he is home and he HAS TO stop whatever he is doing and go pee. If he pees in between, start setting it for shorter increments like 15 or 10 minutes if you have to. If he likes this system, great- it should work fine. If he hates it, that's great too because you can let him know that you will no longer need to do this if he stops peeing his pants and consistantly just goes on the potty!