I am shocked I never say this earlier. The entire time I was watching it, I knew I had not seen it, yet it felt so familiar, turns out this is part of a trilogy and I saw the sequel several years ago.
Anyway! This was fantastic! I was expecting silly '80s trash, but this was a tense, well-written crime thriller. I would compare the tone to _Dirty Harry_ (1971) or _Death Wish_ (1974).
There is a very simple plot here: A teacher starts at a new high school, which is plagued by a violent gang of punks. That's the movie, but there is so much depth. The characters are fleshed out more than I could have imagined.
Not a flawless film, the dialog did dip a little in the second act, but that is more of a nitpick.
Moving onto the cast: we get to see a young Michael J. Fox. Roddy McDowall plays a teacher, in what might be the best role I've ever seen him in. Andrew Norris is great as our lead. The entire gang of punks are all amazing, they each have their own look, and never felt fake. Truly a tremendous job by the hair/makeup, and wardrobe teams for making them so believable.
The two standouts from the members are the leader, Peter - looking like a young Tre Cool - and Patsy. I have an inclination toward punk chicks, and she is a dream girl - so what if she's a sadistic psychopath? I can change her.
So, yeah, I was totally blown away by how good this was.
Gritty, dark, violent, deep, and serious.
I'd recommend this to anyone. Just masterfully made.

Post script: There is a scene where a collection of animals are killed. Thankfully this was all offscreen. The carcasses did look rather convincing, so I hope they were not real, but given how good the effects were, I take it they were fake.
The point being made within the scene is that the gang members are true psychopaths; hurting animals is a common action for those who lack empathy or remorse.