You get creative just like you would at any other job. I'd never say, I quit because my boss is a jerk. I'd say something to the effect of "I was seeking more flexibility, more challenges, more responsibility, more opportunity for advancement, I'm seeking a position where there's more emphasis on the students, more emphasis on professional development, etc. You get the idea. It's all about creativity. There are lots of books out there that will help you find ways of making this sound like it was all your idea and super positive.