The main hallway in the school, is probably the most used part of the game. You gan get to four main places through this area; the Student Council room, the Class Room, the Second Hallway and the Court yard. There is a fifth place you may sometimes get to from this hallway as well, and that is the Teacher's Lounge. But there is no door, you are automatically moved into the room.
Characters who can be found here
You can meet pretty much everyone in the hallway. Although the chances are thin, and characters can be found in other area's as well, there are the occasional times when you can run into pretty much anyone since it is the main area.