Gee, I have no idea of the legality of this situation, but I don't think I've ever heard of a school strong arming a parent on a fixed income either
The only thing I can think of is if you have applied for reduced/free lunches etc. for your son? If you are on a fixed income, I would think you might be eligible for at least some kind of reduced rate. Might be worth looking into if you haven't already.
I sure hope you can resolve this very soon. I'm really sorry you are going through this.....doesn't seem right to me.

sabby