First, I must say that each state's regulations are different - I'm in Mass, where the laws are fairly extensive. We are required to provide all children under age 5 with a "rest" time of 45 minutes. If they fall asleep or not, they still have to rest their bodies. You didn't say how old your son is, so I'm guessing between 3 and 5.
Your first move should be to ask the school for clarification. Sometimes a program doesn't have staff dedicated to potty time, and speaking from experience, pull ups are actually more of a hassle than diapers - diapers can pop on under the underwear and come off after rest without having to completely undress and re-dress the child.
I would open the discussion with the school by saying "My son is potty trained, but I was wondering if you would prefer a diaper / pull up or training pants for rest time, as he has occasional accidents? Or do you prefer extra clothes? I feel he would rest better without having to worry about having an accident." Most programs don't want to go through the hassle of disinfecting the rest mat / rug / immediate area for each accident. And kids have accidents ALL THE TIME.
If you really wanted to get into a certain program, you can technically play the ADA card; a child who has occasional accidents should not be biased against due to physical inability to control body functions when sleeping. (Can you tell I've dealt with those parents before

)
Also, I just want to mention to mommy2savannah, I would be so frustrated to be a parent in your area; a childcare / preschool program should be designed to assist a family, not hinder it by dragging a parent to school to change a child's clothes. It would also be embarrassing (humiliating?) for a child whose parents have to come because s/he had an accident. Just my opinion, of course.