BEIJING - Tanya Davis fled Jizhou No. 1 Middle School one winter morning in March before the sun rose over the surrounding cotton fields covered with stubble from last fall's crop. ADVERTISEMENT In the nine months Davis and her boyfriend had taught English at the school in rural north China, they had endured extra work hours, unpaid salaries and frigid temperatures without heating and, on many days, electricity. Hearts pounding and worried their employer would find a pretext to stop them leaving, the couple lugged their backpacks, suitcase, books and guitar past a sleeping guard and into a taxi. As...