There's a big difference between having fights while you're at school and when you're an adult.
By and large fights at school don't have serious repercussions. You can beat someone up pretty badly and maybe you get suspended from school etc but it's not likely to end up in courts.
The problem with fights as an adult is that either way it ends badly. A lot of guys think right, I'm gonna teach this guy a lesson, but when it comes to the criminal courts you don't have to have done that much to someone to get done for assault, and saying they were provoking you etc isn't likely to help much in mitigation.
When I was at uni my housemate used to bring his mate over to stay with us sometimes who was a law student at a top London uni, he was a sound lad but he did have an impression of himself being hard and he didn't walk away from fights, one of his catchphrases was "mate if someone starts on me proper man, I'll put them on the floor and they won't get up". Anyway the summer after we graduated my housemate found out that this guy had gone to celebrate his exam results on a night out and did coke, there were people squaring up to each other and he couldn't resist getting involved, anyway someone must have started on him and he proved that he could back up his catchphrase because he put the guy on the floor and he didn't get up, because he fractured his skull, smashed his eye socket and cheekbone. Now whilst that was pretty impressive in terms of him showing that he wasn't all talk, the repercussions of that moment were massive for his life, because he had a training contract at a top law firm lined up but instead he went to prison. He came out a few years later but no law career, no graduate jobs coming his way, he's basically struggling around to find work with that criminal record saddling him, it's not just a caution for a bit of weed, he's got to explain on his CV that he served time for GBH.