Can I sue my school for something that happened over a period of time?

Someone can always sue, that's not the issue, the issue is whether or not you can win that lawsuit.  Your case seems to be something that might be very difficult to prove, and is not a "sure thing".  Because of that, it might be difficult to find an attorney willing to undertake all of the time (and expense) of bringing the lawsuit on a contingency fee basis.  If you are willing to pay an hourly fee, then a lawyer might accept the case.   There are many factual issues:  Did you ever complain about shoulder pain while pitching?  Did anyone at the school ever tell you to keep pitching? Did a coach or trainer ever say to you don't worry about how much you pitch it won't bother you?  Did you ever complain about pain?   It seems like you might have many proof issues if you pitched softball for three years, did not complain about the pain while pitching, and then a year after you stopped, you were told you have a problem with your shoulder caused by the pitching.  You have 90 days to alert the school from the time you knew, or should have known about the problem. If you are within 90 days of being told your shoulder is injured, you can still file.  Good luck.