3 1/2 Year Old Having Accidents at Pre-School

*** Adding this: At this age, kids commonly still need a diaper at naps and bedtime. Physiologically. Daytime pottying and nap/bedtime, is a different thing than when awake. Sleep time bladder control, is not something that is even attained until even 7 years old.
Both my kids, still needed a diaper at naps/bedtime, until even when they were 5 years old. My son is 5 now, and wears a diaper for naps and bedtime. He is still wet. Per our Pediatrician, this is common. Boys also are often later.

Don't they take the kids to the bathroom, BEFORE nap time???
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Preschool children do have accidents.
ALL of my daughter's Teachers, from Preschool to Kindergarten to 1st Grade, ALL said unanimously, that kids these ages have accidents.
Sure they could be potty trained, but they are not rocket-scientists about it.
It is childhood. No child, is accident free all their lives.

Or it could be stress.
Or the change in your child's life and routines.
Or some kids are shy to say they have to pee and wait. Then they can't hold it longer. Hence an accident.

All of my kids Teachers, did not treat it like it was a big deal. You just bring extra clothing for your child to be kept at school. The Teacher makes no issue of it. And they treat it in a nurturing manner.

Both my kids had accidents at Preschool and my daughter did even in Kindergarten and 1st grade. NO biggie for the Teachers.

Your child could just be shy.
My daughter was.
She didn't want to "interrupt" the Teacher.
So the Teacher AND I, talked to my daughter so she would not feel uncomfortable and the Teacher, COMFORTED her about it.
Then she was okay.

Have you talked with the preschool teachers? They might make an exception and help out with reminders for a little while. At least until he starts to figure out that he needs to remember on his own. Maybe for school, you can have designated times that he has to go to the restroom to try, like after story time, art, or outside play, and the teachers can remind him for a little while until he gets into the habit of going at the same time every day. Also, I would work on it at home on the weekends as well. Try out not reminding every time, and see if he will start remembering it on his own.

He never tells you he needs to go to the bathroom? He just holds it until you make him go? Sorry, but that is not potty trained to me. He may be "shy" to ask at school but he should not be "shy" at home. What happens if you don't take him to the bathroom at home?
Have you asked him if he likes preschool? Perhaps he is having acccidents because he doesn't want to be there? I think you need to have a talk with him and tell him that he must go when the whole class goes. For now, he has to go as soon as you get toschool (he can go with you) and then when you pick him up (with you as well). Does he do half days? If not, maybe it would be a good idea to start off because if you go as soon as he gets there and then when you pick him up, he probably will not have to go during the time he is there.

It might be stubbornness, but it might also be that his mind might be so busy with school things that it doesn't have time to pay attention to that part of his body. He is still little and can only do so many things at one time, probably one thing at a time. This is something that you will have to really think about carefully.

I was just going to add maybe the preschool could make an exception for him and let him go on his own. I think it is a bit weird that they have to tell someone, when the whole goal is to foster independence in that regard. Maybe it is not set up so it is safe for children to go unattended?

This can be very frustrating, I know. My child does the same thing at church. I would never put her in a 2 year old class! That's crazy to me! His teacher needs to be proactive and ask him if he needs to go. I keep kids and 2 are still having accidents. I make them go about every hour and they get rewarded for staying dry.