3 year old peeing self on purpose in daycare

My son has been potty trained for over a month now. There have been accidents here and there but that was expected. We've really been proud of the progress he has made.Thanks to this they started to transition him to the older class room. He would start with his normal class and then go to the older one in the afternoons. This went on for two weeks.Then on Monday he pooped himself on the playground. This was the first time he did this. Then every day this week he has peed himself and when the teachers talk to him about it he laughs. Today was an extreme case where he first tried peeing on the floor, and when they rushed him to the bathroom he peed on the floor and then started playing in it.We initially thought he was having the accidents because he wanted to be with his old class. The last two days he spent the entire day in his old class and still had the accidents.It's confusing because we no longer have these issues at home. So we're not sure what to tell them to do at daycare. Today they put him in the room with infants for 15 minutes and it seemed to work for the rest of the day, but I'm worried that eventually that won't work.We're planning on trying to send him to school in pull-ups and let him know that wearing undies are a reward. But I'm not sure what else to try. Any advice would be appreciated. via /r/Parenting http://ift.tt/2ygFZux