I taught high school CS for ten years and can tell you from personal experience that you are scratching at the surface of a very promising market. Most schools hand out the traditional lesson plan books at the beginning of the year during faculty meetings and in-services. They are a pita to fill out and maintain and frequently change year to year if you are worth your salt
However, I believe your sales would increase dramatically were you to target administrators and department chairs instead of individual teachers. $20 per instructor is well within their annual budget and they are very open to ideas that make day-to-day operations smoother.
Making it possible for parents to view the lesson plans would enable you to engage entire districts without much difficulty.
Well done.