Okay, so this isn't really about herding cats, but that's what substitute teaching feels like sometimes. Still, here's some advice for people that don't have a lot of experience with large groups of teenagers. If you do have experience with large groups of teenagers and/or substituting, hopefully you'll laugh because it's so true.
When you're a sub, the simplest way to know if a group of students are up to no good is if they're looking at you. It sounds ridiculous, but most middle and high schoolers will never make eye contact with you unless you're talking with them one-on-one. However, if they're breaking the rules they will look right at you to see if you're looking. I don't think they are aware that they do this, but it sticks out like a sore thumb when, out of a class of 28 students that are doing their own thing, a group of three suddenly stop talking and look up at you. It gives them away every time. Even if the culprit (and it's almost always a boy) is smart enough to look away, those around him aren't, and they always go silent which is conspicuous when they're the loudest group in the room. So the next time you see a group of students stop what they're doing and give you the 'deer in headlights' look, stay alert, something is going down.