Yup, I too agree that schools are often soul-destroying. It's one of my bug bears and I bang on about it often. :(
I also agree that some kids, like ours, gain reputations - perhaps deserved - but they are then not treated fairly... 
Maryz, my DS was once arrested put in isolation for pushing a younger child over in the corridor. He was adamant he hadn't. He said he had bumped into him accidentally, then helped him up and apologised. But he was told he could 'confess' and do the rest of the day locked up in isolation, or deny it and stay in there til he confessed.
He did the half-day.
Then he arrived the next morning and they told him he he had to do another day anyway. He walked out and came home, fuming. I tended to believe him, but by then I was battle weary wary, so I phoned to talk to the member of staff who had 'seen' him, to find out what had happened from someone else's POV.
It quickly became clear that this staff member had just jumped to conclusions. I asked him if he'd spoken to the younger child: he hadn't. I asked him if he'd spoken to any of the three friends who were with DS at the time: he hadn't. I asked him if he'd looked at the CCTV footage: he hadn't. He told me he didn't need to because he "knew what had happened". I pointed out that everyone makes mistakes sometimes, and asked him to actually seek some evidence. He refused, again saying he "knew what had happened".
He then tried to intimidate me, telling me they would permanently exclude DS if he didn't 'confess' and do two days in isolation as punishment. I told him that of course I would fight permanent exclusion because I would go insane with DS at home all day, every day when there was actually no evidence that he'd done anything wrong. He told me my son had to report to isolation in the morning or I could consider him excluded, and that the conversation was over!
I went higher, to a deputy head who I'd found to be reasonable in the past, and explained the situation. I asked, and she arranged, for them to review the video and talk to the other children. Everything backed up my son's version of events. If he hadn't had a bloody-minded assertive mum to back him up, he would have ended up permanently excluded. 
Oo, what a rant! I'm still angry about that, aren't I?! 
diet... My son was excluded for two weeks in y9, because he and friends were playing with a lighter, setting fire to bits of paper, in the sports hall one break-time. Stupid sods.
He used to do a lot of walking out, too :(