Actually, I wonder if a simple email referral template might help with nicely measurable word-of-mouth. Plenty of parents will have their kids' teachers' email address, and could shoot them a targetted recommendation with a little help.
I tried giving people a send-to-friend option with a double-sided referral incentive. (i.e. Think like Dropbox: you and they both get something if they sign up.) It was epically unmotivational: only ~1,300 people clicked through to send an invite to anyone (of > 50k users invited to), less than 200 people signed up as a result of invites, and a grand total of two ended up buying BCC.
