LOS ANGELES — Tens of thousands of Los Angeles teachers went on strike Monday after contentious contract negotiations failed in the nation’s second-largest school district. “Students, we are striking for you,” teachers union President Alex Caputo-Pearl told a cheering crowd of teachers marching in pouring rain. Members of United Teachers Los Angeles voted last year to walk off the job for the first time in three decades if a deal wasn’t reached on issues including higher wages and smaller class sizes. Months of talks between the union with 35,000 members and the Los Angeles Unified School District ended without a deal.