I'm sorry you had to deal with those assholes.
Sadly, it starts in primary school and never stops. This is why a lot of women obsess over weight. Women see it in magazines, tv, and then it gets validated by their peers. I can't speak for how men body shame each other, but if it's anything like the normal teasing or what they do to women, it's likely not too nice. Oh and women certainly do it to each other. It's almost worse. A women will cut you down where it matters the most...and usually in the sweetest way possible lol. We are evil.
It's rare if someone speaks up for another until you're at least in your 20's. Even then, there still aren't as many willing to stand up and put "mean girls" in their place. Most people have things to lose. It becomes a balancing act.