(Portland, OR)  --  A Portland Public Schools teacher who was placed on leave for leading an unauthorized walkout is back in the classroom.  School district officials say that Ockley Green Middle School social studies teacher Chris Riser returned to school on Monday.  He was placed on leave March 23rd for an unofficial walkout in which he and some of his students participated in a "die-in" in the middle of an intersection.  Students and parents rallied around the popular teacher, creating an online petition for his reinstatement that garnered more than four-thousand signatures.