Hi,
How old is your child could this be them having some sort of private time?
There are several reasons why a child might spend an hour in the bathroom at school. The first possibility is that there is some sort of a medical issue, and it takes him that long to produce. I'm guessing that this is not the problem here, but it is always important to rule out medical causes for behavioral issues. There are several other reasons why this could be occurring. First, it may be occurring because it allows you so to avoid doing academic work. If you are spending an hour in the bathroom, then you are not spending an hour in the classroom. So, the first thing that the teachers might do is to collect some information about what is going on when your son first asks to go to the bathroom, as this may give you some clues as to why he is doing this. For example, perhaps the teachers will find that he never asks to go to the bathroom duing art or music, but almost all of his requests occur when it is time to do math. If this were the case, it may suggest that he is doing this to avoid doing math, likely because it is hard for him. If the teachers knew this, they could alter his math assignments for a period of time, to make them a bit less aversive, and thus, make them something that your son does not want to avoid. Or, they could let him do math for a short period of time, give him a short break, and start the math again. This may also make the activity a bit less aversive.
Much problem behavior that occurs in schools occurs because it is an efficient way for the child to escape or at least postpone academic demands. However, another possibility is that spending an hour in the bathroom means that he can engage in other reinforcing behaviors, such as engaging in self-stimulatory behaviors, which many children with autism spectrum disorders do. So, suppose a student engages in a common form of stereotypic behavior seen in young children with autism, flapping his hands. What happens if he flaps his hands in the classroom? the teacher makes him stop. What happens if he flaps his hands in the bathroom? He can keep flapping as long as he remains in the bathroom, as no one is there to tell him to stop! Reducing stereotypy is extremely difficult, but the place for the teachrs to begin is to make sure that there is more reinforcement available for staying in the classroom and working than escaping to the bathroom. We all need reinforcement to maintain appropriate behavior--for me, it is my paycheck! For your son, it may be access to specific toys. it will be importantto find things that will motivate your son to keep bathroom trips to a reasonable amount of time.
I think setting the clock may be a good idea, but you might combine this with some extra reinforcement for coming out when the clock sounds. Now, if the teachers set a clock and he comes out, what happens? Play time ends and work begins! The teachers might consider setting a clock, and then developing a program that if we exits the bathroom within x number of minutes (perhaps 5 minutes) after the clock sounds, he gets a reinforcer, such as playing with his favorite toy for 5 minutes in the classroom before he starts working again. Eventually, when he is successful, you can start to alter the times--he needs to come out within 4 minutes to get 4 minutes of play, he needs to come out within 3 minutes to get 3 minutes of play, etc. The key will be to make it worth his while to come out, and then when he is doing so reliably, then make the criteria a bit more stringent. being out of the bathroom has to be more fun than being in the bathroom!
I hope this is helpful. Best of luck.