THey have designated potty times at Savannah's school and they take kids when they ask or if the staff member sees signs that they need to go. Most of Savannah's class is still using diapers or pull ups to some degree so they still have a changing table in the class, but she's still in a toddler class.
Everetts teacher has them go potty at transition times (before going outside, before/after meals, before/after naptime.) and if/when they need to go. The classroom has a half door that goes to the bathroom area and a window (about 4 ft high) that she can peek into the toddler bathroom for the more independent kids.
Yes, they have times when they go to the toilet ie before snack, before lunch etc but I'm sure if you let them know your LO is in the process of PT, they will ask him as much as possible if he needs to go.
when he was in the older infant room, they did potty training at diaper change times (and of course when they said they have to go). In his new preschool room, it is pretty much when they have to go they tell them. But our schools is really good about working with the ones who need to be reminded.