AP Photo

The headquarters of what has rapidly become the largest school in the world, at 10 million students strong, is stuffed into a few large communal rooms in a decaying 1960s office building hard by the commuter rail tracks in Mountain View, Calif. Despite the cramped, dowdy circumstances, youthful optimism at the Khan Academy abounds. At the weekly organization-wide meeting, discussion about translating their offerings into dozens of languages is sandwiched between a video of staffers doing weird dances with their hands and plans for upcoming camping and ski trips.