I believe Mr. Pash fabricated the "assignment" in advance to divert suspicion from himself. Roger is unable to defend himself, after all. Roger's parents did say he told them he had seen something very bad at or near the school grounds shortly before his disappearance. He also said he was...
The stairs leading to the side exit were right next to Roger's basement locker. This side exitled out to a windowless side of the building , 300 feet from person of interest John Pash back door.