6 year old son still pees his pants at school

My 6 year old 1st grade son still has trouble wetting his pants while he's at school.  4 times last week when I picked him up, I saw that he was wet.  At home, I've developed a remedy.  I set a timer for 30 minutes and every time it goes off, he uses the bathroom.  It's worked very well but he doesn't have a timer at school.  He keeps saying that he tries VERY HARD to not pee but I don't see any improvement while he's at school.  He also still wets the bed whenever he wakes up.  I know he will probably grow out of both issues but its VERY frusterating in the meantime.  Any suggestions?  Reward system?  Positive reinforcement?  Anything?