My 4 and a half year old has started peeing his pants again?

This problem just started in the past 3 weeks since he started going back to preschool again after New Years. He has a new preschool teacher who started right after the New Year, old teacher's last day was the last day before Christmas break started. When before he would almost never wet himself with the old teacher now he comes home from school having wet himself anywhere from 1 to 4 times each day during the 5 hours he is at preschool! Its quite frustrating because he doesn't do this at home but will sometimes do it when we isn't inside the house, like if he is playing outside or at karate class. Any suggestions to stop this? They have him on a potty schedule at school and I try to keep the same potty schedule at home. I frequently keep him naked from the waist down now at home to avoid accidents, maybe he's having more accidents at school because he has underwear and pants on? I give him a sticker when he uses the potty at home but not sure how else to motivate him to use it? The problem only seems to arise when we are out and about or when he's at preschool. Please help! I want to put an end to this wetting because I know he does know how to use the potty.