In 2015, a young engineer from Kerala in south India came up with an ambitious plan to take math and science online and make it fun for the country’s stressed teenagers. But instead of remaining focused on education and technology, Byju Raveendran appended a gigantic sales machine to his edutech — with Bollywood superstar Shah Rukh Khan and soccer legend Lionel Messi as his brand ambassadors, and an army of reps hustling to