So since this has been broadly appealing but with a few problems, like the teacher's inexplicable response to the charge of bullying, I set out to find and read the original. And did.
It was first published as a 30 chapter web novel, then adapted into a one volume light novel. I don't think the LN has been translated - at least I couldn't find it - but the WN has been, so I read that.
And what follows is going to be very mildly spoileriffic, but since it "spoils" broad story beats that are obviously predictable in a story like this, and addresses some broad misconceptions that are interfering with some readers' enjoyment, I'm just going to forge ahead.
Specifically, the teacher's actions are given more context in the WN (though not a lot more and it remains awkward storytelling). Basically, she's fresh out of school, something of a crusader and pretty much an idiot.
And notably, the attitude later in the story is that she fucked up and the class knows it. And she knows it.
And there's a detail that's significant that IMO didn't get enough emphasis either in the original or in this adaptation. I've seen that people are saying that it's dumb to think that someone would believe that the quiet girl would bully the "extroverted" girl, but that's not quite it. It's more accurately that the pretty bookish girl would bully the gross otaku. It's still not fully credible (which is part of how and why the teacher's an idiot), but it is at least marginally plausible.
The overall theme that comes out of all of this is the tendency among people to rush to judgment with insufficient grounds, and that's revisited more than once in the story - this is just the first example of it. And yes - it's a bit awkwardly contrived, but that's just the way it goes.
Broadly, I want to make the point that, having read the WN, I intend to keep reading this adaptation. I enjoyed it all in all. It's not world class storytelling, but if you just slow down and cut the author a bit of slack, it's worth it.
Or, of course, if you're so inclined, you can rush to a negative judgment with insufficient evidence.
Like the teacher did.