Check your local laws. If he's in a 3-6 classroom, you might be able to say that he should be bathroom trained before entering the classroom.
If he's 3 and giving you problems with changing a diaper, time to talk to a physician possibly.
MattMontessori school questions: 1. How to change diaper of a 3 year old, creating trouble for change of diaper?
One more time educate yourself regarding children
get out of the preschool you seem to be clueless
around children.