I teach secondary science- I think there are really good things about the job. It's certainly never boring! And I get to talk about my passion all day. I think the kids can see through you if you don't care about your subject, so it's worth teaching something you really care about.
Workload is a big issue, some people can set firm boundaries around this. I'm not so good at it, and I'm also very prone to getting sucked in by the very needy kids- the ones who need extra support and help to do well. Which is not a bad thing, obviously, but it can mean you invest a lot of time and energy in one student, who potentially just isn't capable of succeeding at that moment in time. I'm trying to do less of this, but it's very hard!
I'd say in general the kids are "fine"- but maths is often a subject they struggle with, so behaviour can be more challenging. Morseo than behaviour, I'd say the big issue is mental health and social issues- these are a massive problem in teens and there is really limited support out there. You have to learn to protect yourself emotionally a bit. I was really bad at this in my first few years of teaching, I'm still not great at it, but if you get really sucked in, it will honestly damage your mental health too, so try to protect yourself where you can.
In the first few years especially, I'd also add pay isn't great. M1-3 is less than the national average full time salary, and given the hours you are working, it often doesn't feel worth it. But by the time you graduate, pay may have gone up to reasonable levels! And as a maths teacher you can sometimes negotiate going up the pay scale a bit faster. The pension is good, but don't forget it comes with big deductions.
Schools are very different from each other, so I would say if you find you don't like one school, it's fine to move. However, it is a fact that most state school maths departments are currently short staffed, and this creates extra workload, and may be hard to escape.
A maths degree will give you loads of options though, so if you find that teaching isn't what you'd hope it would be, it won't be wasted!