Bridge International Academies, backed by investors including Bill Gates and Mark Zuckerberg, has opened more than 400 schools in Kenya in a bid to bring inexpensive, private education to some of the world’s poorest children. In rural Kenya, 6-year-old Sharon Ndunge, sitting in a rough-built classroom with chicken-coop wire for windows, a tin roof and wooden benches, is among 126,000 students enrolled at the more than 400 Bridge International Academies that have sprung up across…