I agree with Patti and the other posters here. Being a sub (Plano ISD) teacher, especially in the beginning of the year, the teachers are setting up the guidlelines for class behavior and rules throughout the year. Sometimes teachers come across a little tougher because some students think they can preside over a class and you have to nip that fast to let them know who is in control.
My experience has been that when we take the children to the gym for PE, we stop at the water fountain and restroom. The Coaches prefer the children not leave until all the warmups, etc are done. I have never seen a coach tell a child they could not have water. Most of the time there are 2-5 groups of children in the gym at the same time (up to 90 children or so)
What we do see, are the children who have no interest in PE (or regular class for that matter), therefore they have every excuse in the book for going to the bathroom, water, nurse, etc. The rules in the beginning of the year, sort that out a bit and then as a teacher you know who is trying to get out of class.
If you are still concerned, I would certainly make a phonecall, email or visit the coaches.
Good luck with your school year!!