In a development that would have been unheard of a decade ago, about 200,000 U.S. school children are enrolled in full-time online programs, BusinessWeek reports. Eleven years after its founding, K12 has 81,000 students in 27 states and the District of Columbia. If it were a school district, it would be one of the largest in America. K12 expects to generate $500 million in revenue this year–it earned a $21.5 million profit last year–and its stock has doubled in value since the company went public in December 2007…