For giving teachers an app to help them solve their biggest problem—classroom management—through real-time feedback and online rewards. The cofounders are Sam Chaudhary, a former teacher and education analyst, and Liam Don, a game developer who turned down a recording contract to pursue a PhD in education technology. In its first year of beta testing, the site attracted more than 3.5 million teachers and students in 30 countries and raised $1.6 million in seed funding from investors including Learn Capital and Paul Graham of Y Combinator. The best part is that most of the growth has been viral—word of mouth among teachers over Twitter and Facebook—showing that the program has real traction with its target audience.