It is not illegal for a teacher to be an airhead.
Write a letter of complaint to the principal and the Board of Education. It likely won't have any effect on her career (unless lots of you complain), but it will probably make you feel better.
In the future, keep spare copies of all of your assignments. My son has a similarly airheaded teacher this year who apparently thought her students were not going to fight back. Her reputation preceded her, and the kids all had a plan to deal with it. It's very hard to argue with a student who produces the assignment again immediately from a time and date stamped CD.

