ATLANTA — As a generation of teachers retire and their burned-out younger colleagues quit sooner, complaining about the workload and the pay, public schools have struggled to keep their classrooms staffed. Georgia lawmakers thought they had found a solution, passing House Bill 385 three years ago. It allowed retired teachers with 30 years of service to return to the classroom with full pay and pension from the Teachers Retirement System (TRS), provided they sat on the sidelines for one year.