Are you kidding? Did you go to school? Teaching material to a class of 10 (let alone 20 or 50) K-12 kids, selected only by location and socio-economic class, is a ridiculously overconstrained problem. To give one of the main problems: for each concept you teach, you have to choose how long to explain it and give examples. If you move on, then any kid who didn't really get it will become very lost for the rest of the year (I'm thinking of technical subjects, where you have long dependent chains of concepts). If you keep dropping kids, then everyone gets lost. If you wait until everyone gets it, then you go absurdly slow. My little brother has been "learning" basic arithmetic in his (small, private) school for six years.