I would try again to speak with his teacher. The school nurse is not in the classroom to see the individual circumstances when this is happening. My daughter, never having had a potty accident since she trained at 2 yrs old, had an accident in kinder. I had talked to her until I was blue in the face about not holding it (she would get in the car at pickup time DAILY at 11:45 and immediately HAVE to go potty--good thing we lived 5 minutes from school). A few times, I even had to just park the car and walk back into the school. She just didn't go at school, and to that point, she had managed to make it/hold out. But one day, she needed to go when she was in her reading group. It wasn't her turn to read so she was supposed to be quiet and not talk while another student read. She did. And wet her pants in the process, instead of raising her hand or interrupting b/c she didn't want to "break the rules". She was a very rigid rule follower.
I had to talk to her at length that day, and immediately afterwards IN HER PRESENCE talk with the teacher, and get the teacher to reiterate to her that it was ALWAYS OKAY to go to the potty if she needed to. It was right in their classroom. No other student had to go with her, it was no inconvenience to anyone else, she didn't even have to raise her hand and ask. Straight from the teacher's mouth, with me as her witness. She never had another school accident.
Now, maybe your son doesn't have the same "rule" issue my child did. If they are all expected to go and he doesn't follow suit, then he must not mind breaking the rules. But perhaps there is something else that is equally glossed over that makes him not want to go? Does he have privacy issues? Does he get "stage fright"? Is there something about the restroom at school that he doesn't like? Does it smell badly? Does he not like the tissue? The hand soap? Does he have an issue with the doors or the "locks" on them? There could be a bazillion reasons besides him just being obstinate. Talk at length with him and see if you can figure it out. He may not even know exactly what it is unless you happen to word your question just right.
But I think you should talk one on one with his teacher and let her know you are trying diligently to figure out the problem and address it, and ASK HER if she has ANY ideas or suggestions.... Then see if she is willing to work with you and your son to get through this. Maybe some sort of signal so he doesn't have to ask in front of everyone? Maybe let him be first in the line of kids when it is time for them to all go? Or last? Try different things... maybe you'll hit on it.