It looked like a normal first day of school at Patria Libre elementary. Uniformed kids sporting brand-new backpacks with their favorite cartoon characters — Dora the Explorer, Hello Kitty, the "Frozen" heroines — reunited with classmates and sang the national anthem. But that's far from normal in Oaxaca, a Mexican state where teachers' strikes and protests cost the average student 50 days out of the 200-day academic calendar last year, according …