1) A portion of teachers will spend money on software. Reports of teachers being universally poor and stingy have been greatly exaggerated. You may know a stingy teacher. I do, too. I also have credit card numbers willingly given by thousands of her coworkers.
2) Average sales cycle for districts is 9 to 18 months. You can close a teacher in 9 to 18 minutes a non-trivial portion of the time.
3) There exist many, many businesses which have sold more $$$ to teachers than BCC. 5,000 paying customers is rather modest, as there are over three million teachers in the US. I'm always happy to be mentioned but please don't think I'm the upper bound for success here.
4) Contra article, making money is a perfectly valid reason to go into education. Teachers get paid on Friday, too. Also, some judicious capitalism would help redress the severe resource misallocation between education software, which is probably societally important, and mobiphotosocialgames. (We could also quibble with the misallocation between education software and education salaries, but that will not endear you to the main customer group here.)