M.C.
I need clarification on something. So he goes by himself at school, but not at home. Does that mean that he wears big kid underpants to school, but when he's home he wears a diaper?
The only way to make him go consistently is to be consistent. Throw out the daytime diapers. ONLY use a night time pull up for bed time. As soon as he wakes up the pullup comes off and big boy pants go on.
My son was 3 1/2 when he trained. My brother was close to 4y. If he has shown that he can go and stay dry at school, then it is being lazy.
Talk to the teacher, find out what routine they do at school. Use a similar routine.
BTW, my daughter LOVED the Potty Power DVD.