My Son At 3 Months How Do I Get My 3 1/2 Year Old Son To Go Potty At School?

How do I get my 3 1/2 year old son to go potty at school? - my son at 3 months

My son is 3 1 / 2 years and trained at home and everywhere Bathroom (business, church, houses of others, etc.), except that it does not go to the toilet in the preschool age. There must be a pull-up at school and at home, use only takes underwear, even during the night. If one goes to school in his underwear that he kept the whole day, or with many accidents and was very different from what they just rely on the Pull-ups interfere with the next day. Please help!