It's a commonly used term in self directed schools, it basically means that while you're supposed to be spending periods in class, you simply walk around in the hallways. The only time Hallwalking poses a real threat is if a person of direct authority (principal, TA, VP, etc.) catches you, in which case you could get detention. In conjunction with hallwalking, people take "laps" around the halls.

(noun) A successful type of theft which results in an acceptable, impressive and rewarding payday for the protagonist.