injured during school

Since there is no question here I assume that you are asking if you might have a claim against the school for negligence. Depending on the circumstances of him slipping in the bathroom, you may have a claim. It will depend on whether the school was negligent and failed to notify students of a condition that would cause injury such as water on the floor or some other reason for slipping. Not sending him to the nurse/physician immediately would be a good indicator that they failed to take adequate precautions, failed to provide him with adequate care and may enable you to make a prima facie showing of bad faith in reporting the problem.
In short, you may have a claim, but it depends on the circumstances and facts of your son's fall.

I'm afraid I'm less optimistic as I cannot think of anyway to hold the school liable for anything that happened under the few details you shared. But, in case I'm wrong, please know that the deadlines for asserting claims against governments are very, very quick in coming -- so I'd be very diligent in finding an attorney who is more hopeful than I am.

However, if somebody other than the school -- such as another student -- was responsible for the fall, that may be a better avenue to explore.

Good luck,
Marc