If you want honest answers, ask your child's classmates/friends. You don't have to mention that he peed. Just say hey, did Johnny ask to use the bathroom twice and teacher said no? Also, where would your son get extra clothes from? Why weren't you notified of the accident? It's not paint on his clothes, it's bodily waste. I'm sorry but I would be pissed

Please call or email the teacher to hear what happened from her. My son said boys couldn't use the bathroom in the room last year and had to go in pairs to the hall to leave it available for the girls. Even from his story, there was definitely more to it but this never became an issue so I didn't have to ask. My son also admitted asking frequently to get out of work then taking a long time. I'm sorry this happened to your son but please get the entire story from the teacher before going to the principal.

Is it possible some students in the class have been asking to go to the bathroom a lot as an excuse to fool around so it's hard for the teacher to tell when it's a real request and when they just want to make trouble? Is your DS in particular someone who has done this in the past? Could be a crying wolf issue? I'm not trying to defend the teacher but trying to figure out what was going on in her head.
Of course she doesn't want an accident so she must have had some reasoning.

I'm sorry OP. Your son must have been so upset. I would definitely talk to the teacher and the principal if you feel you get the runaround.

Our MCPS school has a policy that children can go to the bathroom whenever they need to. Many years ago I requested that all school staff know my son should be permitted to go frequently due to a medical issue. The staff assured me that all students were allowed to go at all times.

This happened to my daughter's friend. The teacher said no going to the bathroom. That ended as soon as someone wet their pants. Is the teacher new? I think it's cruel to do that to kids.

A second grader purposefully flooded the boys bathroom at my child's school earlier this year. I think some rules about using the bathroom were changed as a result.

I mention this anecdote as a possible context for something similar (new rules) in your child's classroom/school. I can't imagine no bathroom breaks from 8:20 until 11:05 - that seems unreasonable and counterproductive. But perhaps the rules did change and that's what you need to inquire with the teacher about and then convey to your son.

Good luck.