getting on disability through the school employees retirement system

I worked for the school system for 15 years as a headcook there was a lot of heavy lifting and walking on concrete floors, last March my Dr. took me off work I had some health issues and was in continuous pain with arthritis in my spine and through out my body plus two knee replacements. I filed for disability through SERS ( school employees retirement system) and was turned down, they felt that even though I am almost 59 years old I should still be doing that work lifting 50 pounds and all the other heavy work that headcooks do.I am appealing their decision but they said I can only appeal it one time is that correct? This is not SSI it is SERS.