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.
1 - It's period three, let's go hallwalking before lunch begins so we can get lunch early.
2 - I need to talk to you, let's take a lap. *5 laps later* just one more lap!