From pkcourtesy.blogspot.com
I realize that some teachers, or professors, think they're better than others. They think that somethings are beneath them. There's nothing quite like walking into a classroom and being faced with the board covered in writing from the previous class. Now if teachers can't be bothered to erase the board then they should assign a student to erase it.However, I think that if the teacher made the mess, they should clean it up. Not erasing the board after you use it is similar to not flushing the toilet after you use it. Just do it. It's common courtesy and not beneath you.