As the resident "Been there, done that" guy with regards to teachers I think I'm 99.9% of the way to "You should probably build something for businesses instead -- for equivalent levels of savvy/work you'll get a lot more money while suffering a lot less."

Teachers: $$

Businesses: $$$$$$$$$$$

I just got an email this morning from someone responding to an AdWords ad for Appointment Reminder. The ad costs $8 a click, for reasons which will soon be obvious. The contents of the email: "Quote me a price for X appointmnts a month." I did. "OK, we want it." If they stay around for the year I just earned several hundred copies of BCC with two emails and a few minutes of data entry.