STNA or MR/DD direct care worker

I have been looking for STNA training classes and while doing so I have stumbled upon the direct care worker for the mentally disabled. I wanted to find out if there is anyone that has worked as a STNA or a direct care worker with the mentally disabled. What are the disadvantages or advantages of working with the mentally challenged or the elderly. I am looking for a positon that will allow me to be flexible with my school schedule. I think working as a direct care worker would do that for me. I heard working as an STNA can be very hard, b/c no is willing to help you so I have been told, but that could have been just one person. I don't know if you have to work a standard shift of if you can pick your own hours.

I plan to quit my full time job, so that I can concentrate more on school, but I want to learn and enjoy my part time job whatever it will be.