My DD2 was in the same situation. She napped in a pull up every day up until Aug 2015. That's when she moved up to the Pre-k room and no pull ups were allowed. We just hoped for the best knowing they would make sure kids went potty right before nap time. Turns out she stayed dry more times than not!!! She is still not night-trained even though she's been day trained since 2. But this entire school year she's had less than 10 accidents during nap time.
I'm sure they've had this situation before and know to expect some accidents... just make sure you have plenty of extra clothes and good luck!