Obviously, you want to always have your boss in your corner, but this may be the exception because it is dealing with your future. Aren't your a member of the teachers union? It may be in your best interest to get your union rep involved to do the fighting for you. I am sure that you signed a contract to teach and it is the schools' obligation to honor the contract and the American with Disabilities Act. If the lifting is causing you distress, then ask the principal to come and do the lifting for you or provide an assistant to do the lifting. If the principal does not think that hiring an assistant or his helping with the lifting is ok, then remind him of your request to switch over to be a 2nd grade teacher. Most younger grades have home room mothers that volunteer, thus providing you assistance if you need to leave the classroom to use the restroom. Again, if all else fails, contact your teachers union and let them help you.