1- send another check
2- try to figure out your check number (they are usually numbered, look at the first check in your booklet if you have one, the check you sent should be 1 number less)
3- if you find the number, call your bank, tell them the number and that the check was lost (I actually think anyone can cash it, not 100% sure though)
4- if you see that it has been cashed in by someone other than the school, call your bank immediately, they should refund you the money