Actually, my son gave up his nap at that age. He also fell asleep in the car as I feel it is natural due to the motion.
When my son was in daycare - some children were not required to nap, just rest or offered something else to do. I feel when you speak to your son, you should get down to his level and try not to be overly authoritative. This is where many parents have things backfire. Not to mention - it is the age and the stage.
If this is his behavior, then I would assume he is ready for something else at nap time. Also - if he is inquiring about 'peeing'; he is interested in potty training, if he is not yet trained entirely. Or he is just repeating what he heard the teacher say.
It is the caretakers' job to help guide your child as well while they are in their care. You can talk to your son, but at this age, he is really not going to recall your conversation in its entirety. The 'teacher' also has to own up and take responsibility as well. If the other kids want to do what he is doing, then in all honesty - I really do not see the issue if they are the same age... or she should put him in a different room until the rest are asleep and then offer him a choice of projects. This is what they did with my son at daycare. (they had a few different rooms with different schedules so it worked.) It sounds as if she is not really trying and she is putting it all on you. You are not there, she is.
If he still takes a nap for you on the weekend, then so be it... but do get ready as he may just give them up very soon. When my son gave up the naps, we just put him to bed a little early and he fell asleep really fast. It was great!