Old-school means attached to an older way of doing things: old-fashioned, or belonging to ideas and styles from the past. It can be kind ('old-school manners', or old-school hip-hop meaning the early, respected kind) or a gentle tease (he's a bit old-school about phones). Though actually, it does not always mean worse or out of date; often it means the earlier, stricter, or more original kind, and the speaker likes that. It is informal. You will see the little line in old-school especially before a noun. It is not about a school building that is old.