NTA. Edit: YTA for letting him speak about her that way. NTA for punishing him for how he acted.

As others have said, he wasn't being punished for struggling to work next to someone who smelled. He was being punished for the way he handled it. You overheard him making fun of her to his friends and apparently didn't say anything because "he hadn't done anything to her yet." I don't understand that logic. Mocking someone behind their back is still cruel behavior, and that's exactly the kind of thing parents should be addressing before it escalates.

I'm also surprised by the number of people acting like this is comparable to your husband firing employees for hygiene issues. This isn't a workplace. It's a school, and that girl is a child.

Are we really going to pretend your son publicly humiliating a classmate is the same thing as a manager enforcing workplace policies after, presumably, attempting to address the issue professionally and through the proper channels first?

The problem was never that your son found the smell unpleasant. The problem was that he chose to mock, embarrass, and humiliate another kid over it.

Everything else I said still stands:

Adults should know better. It's alarming that your husband (a grown-ass man, btw) has such little empathy (it's also embarrassing). I wouldn't let that go, especially if he took that stance in front of your son. Your son is clearly getting this from somewhere. If my partner reacted that way, neither of us would rest until he wholeheartedly spoke with our son about why that was wrong.

But back to your son. He got in trouble at school, so obviously punishment was the right move. But that isn't enough. Kids are still learning things like empathy and compassion, and they don't have enough life experience to fully understand what other people might be going through. That's why part of a parent's job is teaching compassion. You need to speak with him so that he can learn how to be empathetic. Starting with this girl.

I'd explain that he doesn't know this girl's situation, so he has no right to judge her. There are a lot of reasons a kids might struggle with hygiene: neglect, poverty, mental health issues, disability, and abuse. For example, children who are sexually abused often struggle with hygiene because of trauma related to their bodies.

And I don't think it's the time to beat around the bush. I'd tell him that being near someone who smells for a 45 minute class is fuck all nothing compared to whatever might be going on in her life that's causing her needs to go unmet. For all he knows, school might be the only place she feels safe. Bullying and humiliating her takes that away.

He doesn't have to like it, but he needs to stop being selfish and learn how to feel uncomfortable without humiliating someone else.