Only because "profit" is in the way. $20K a year with 20 students is $400K per year which is more than enough for two teachers and the infrastructure and then

Only because "profit" is in the way. $20K a year with 20 students is $400K per year which is more than enough for two teachers and the infrastructure and then some. While this is fine in elementary and early middle school (which is where the Gates foudnation documented), this probably breaks down as you enter high school where teachers need to be specialized and you need to have a pool of teachers to cover all subjects.