Los Angeles teachers on Monday walked off the job Monday morning in their first strike in 30 years, placing hundreds of thousands of students in limbo. About 31,000 members of the teachers union had agreed to walk out to try to win smaller class sizes, more support staff at schools and better pay. Thousands of picketing teachers and other union members — many of whom had arrived at their schools before sunrise — joined the midmorning downtown rally in the chill rain along…