How about an air horn? One short burst should quiet everyone down for a few seconds.

When confronting someone, think about what the other person wants, and then think of what you want. Example: Students are allowed to go to lunch early. You can say ‘I know that you like going to lunch ahead of time, but that’s not going to happen unless you stop disrupting other students by being loud in the hallway.’ Chances are, since the students really want to go to lunch early, they’ll listen to you. The scenario changes, of course, because I don’t know what students in your school want. This is just an example. Just be nice and sincere, and don’t put on the bossy attitude, because that’s just going to backfire.