OP, I don't know why. It didn't work when I was in school, and I can't imagine it works in the majority of schools now.
Especially since kids are taught not to "tattle", which many adults define as "letting an adult know that someone has done something wrong", a kid can be a bit confused about when they are supposed to tell adults things! Heck, on the DLR forum we had someone not want to tell a CM about a family that was doing something wrong, because they didn't want to tattle, just like they'd been taught in school.
And if the people doing the bad things are the "cool", popular kids, NO ONE is going to tell, and even if they did, the teachers and administrators (at least in the San Jose Unified School District from '75 through '87) won't BELIEVE them, because those kids are great and fabulous and wouldn't do that! Cool kid sprayed Binaca (does that still exist? "hot" minty breath freshener) in my friend's eye at a school dance...he was NOT punished even though my friend's night was ruined by an incredibly painful injury. He did it on purpose. 8th grade trip, I made an innocent mistake on where to meet the group, I got punished by missing the rest of the day's sightseeing (had to sit in the bus) when the cool kids (whose parents chaperoned) were sneaking out of the rooms at night, being seen by the drunk chaperones (we saw them before it was time to go to the rooms), and were not being punished. etc etc etc etc.
The non-popular kids aren't going to tell if it's the popular kids doing it...even if they would be believed...
Peer pressure works wonders at that age. I taught middle school for almost 20 years and though I tried to avoid punishing the whole class due to a few, at times it was the only way. After a few times, the others would get the few on the right track or rat them out. It isn't my preferred way, and I know some people use it to the extreme.
You probably had a unique class situation. Or I did. Alas, only way to tell is after the fact, after the damage has been done!
Peer pressure works when it's the PEERS choosing what to pressure others to do...not when the adults choose it...
And, once, again, the overall tone of several posters on this thread is that the teachers/administrators are out to get you and your child. How sad that educators are seen so often as "the enemy" today.
It's not just today...
When everyone pays for one person's stupid action, there is no incentive for doing what you're supposed to be doing even though other people aren't.
It's worse than that. Some kids will figure if they are being punished for something they didn't do, then they may as well go ahead and do what it is they are being punished for!!!
Although not everyone will react as extremely, the above is just what happened to DH. This is an at-home thing, but it has colored his whole life. At 6, he bought a candy bar for his mom at school. DH has always been a bigger human, well, at least since toddlerhood. His mom had him on a diet starting at 3 (though her version of a diet is a bit odd). It was hard for him to not eat the candy bar, as he wasn't allowed real sugar like that (he got Tab while his family got Coke, even at that age). But he did until the bus ride home, when some friends took bites out of it. He was 6 and thought that the only thing to do was to wrap it up and hope mom didn't notice. She did. And so did dad. Dad called him a liar, didn't believe his story. DH got punished, by old school parents in old school ways, for lying (parents could have asked his friends, but they didn't even have the decency to do that). And he remembers at THAT moment, he realized that he was going to get punished for things he didn't do...and therefore, he should do them. Started a lifetime of sneaking and lying for as long as he could without being caught.
NOw he's an out-lier...not everyone is going to go to that extreme. But plenty of people do get tired of being punished for things that other people do, when they have absolutely no control over those other people...very very demotivating...
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There was a high school in our area that caught kids drinking and kicked them off sports teams (per the agreement they ALL signed, parents and students, stating they would stay substance free during the sports season--all athletes in MN sign this) and a bunch of parents tried to sue the school.
Slightly related.
A few years ago, at a HS here in Tacoma, the football team was on its way to the championships (or whatever they are called). They had a few serious STARS on the team, amazing players. They felt they could do no wrong.
The coach made sure that everyone knew they *had to be* at the practice before (or the team meeting? not sure...eta, couldn't have been the DAY before, not enough time). If they weren't there, they weren't playing in the big game.
These kids didn't show up.
The coach told them that they would be cut them from the game.
And....almost the entire community...rallied...behind the players.
The coach ended up quitting. Retired, actually. He'd been the coach for that team for something like two decades, and no one backed him up on his good decision, because it would cost the team the big game. The administration stepped in and denied his decision.
It was awful. What the heck did those boys learn from that? Are they now in college taking bribes from the schools, getting money and cars...will they have the decency to give back trophies they earned while breaking the rules? Or will they think it's OK? After all, the whole city decried the coach for his decision to followup on the rule...
It all sucked. Coach is the father of my good friend...I heard most sides of the situation.