1. even if they're all watching porn, what makes you think they're watching porn of their teacher? it seems very likely none of them would ever find out
2. even if they see that porn, it's awkward but there's no reason to actually punish the teacher for it

If it comes up in the school, in an ideal world the administration would discuss it with the student(s) if necessary, and implement material about sex work for the sex education

Even in that world you'll need an impenetrable skin, teens will be teens and bully mercilessly any weakness the adults show

i would be be far far more worried about the harassment a teacher would receive from their students than any impact on the students.

you deal with it by not tolerating bullying, which might still be a challenge for schools lol.

Same as anything else, really. If they say or do something that's inappropriate, you sit them down and have a discussion about why it was inappropriate. Use it as an opportunity to teach the child the right way to treat other people. That's the whole reason why children have adults in their lives, to teach them right from wrong and protect them from harm. Children who are old enough to actively search for sexually explicit material probably aren't going to be harmed by viewing the sexually explicit material itself, but if they're viewing sexually explicit material of adults in their lives, then they should understand it would make those adults uncomfortable, so if they're going to look at porn they should stick to (consensual) porn of strangers (obviously with the caveat of "porn isn't representative of real sex and can be problematic in its own ways" but that's its own discussion).

You're going to get a lot of hopeful, but naive answers from people that mean well, but are arguing about a perfect hypothetical world.

What will happen in the real world with immature kids, is that some kid will find the pornographic material of the teacher, send it to their classmates, and the class will then proceed to relentlessly mock the teacher. There will be no respect and the situation will become untenable. The school will then face the choice of having to disipline dozens of students for inappropriate behaviour, which could involve multiple expulsions depending on how far the mocking went, or the school fires one teacher. What choice do you think most schools would make in such a scenario? I'd pick disciplining all the students and expelling those that leaked the pornographic material, but we all know most schools would not do that.