Does he use the toilet in school, or just go in his pants? If he is able to just go in his pants at school then I am sure he is getting confused as to why it is okay in one setting, yet not in the other. Our friends had this issue with their middle son when he was in daycare. The daycare provider refused to help with potty training because she felt she had too many children to focus on, so once the parents would leave she would take the boy out of training pants and put him back in diapers. The parents could not figure out why he kept wetting himself, until they showed up early one day and found their son in a diaper. They ended up switching providers so that he could have stable training no matter where he was.
If you are worried about him having accidents in school pack some extra pants with him so that if it happens he will have something dry to change into, but I would use the same methods both at home and at school.
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