Good move on the social DRM decision. I'd suggest looking at ePub, which is rapidly becoming the most widely supported format. It will allow your pages to reflow on different sized screens, which PDF won't. That way, if a user can't afford the most optimal reader, e.g. the V9t, they can still read your content on whatever they do have.
Regarding the educational market, I'd suggest that you still consider social DRM, with the name of the school or even student, along with the date. If you make it simple for people to refresh copies from your server, it will be easier for them to pay the appropriate fees for doing so, and continue to get updated content, than to pass around files at the school. And I think students and parents will notice if their kids come hope with books that say "Personal property of Ms. Jones - do not distribute" (principal's name) on all of them. Schools that are still willing to distribute files marked in this way honestly probably can't afford your content anyway. Maybe you could have some kind of grant program for schools to demonstrate unusual financial hardship. (All public schools that I know of have financial hardship-- but some more than others.)