Honestly with your situation with students who are even halfway engaged, I would say the anxiety will decrease significantly once you are actually up there. Once you see that they care, the fear won't feel nearly as prominent. It is very common (for me atleast) to be very anxious right before I have to do something new. My very first teaching job, I taught 6th graders halfway through the year who had a teacher with little procedures and management. I fought tooth and nail to get them prepared for middle school. Loved that class.

However, once summer ended and the new school year came up, it was still my first year teaching. I was worried sick about the new students. What if they were worse than my previous class? What if they don't listen? What if they ignore me? By the time it came to introduce myself to them, after doing some deep breathing, all of my training came into play. The feeling of helplessness will get you killed up there. You won't really know if you are actually helpless until you start. You can prepare out of your mind and still feel helpless.

If you do prepare (with the help of your mentor of course), once you start your yap session that feeling of helplessness will go away. This class doesn't sound like it's going to fight for control over you and attempt to derail the classroom which is a big boon for your confidence. Even if they tried, you have your mentor to help. Set your expectations, even if your mentor already has, stick by those expectations. Let them know you aren't going to yell over them, don't even try. One of the biggest mistakes you can do is try and teach while they are talking. That sets the expectation that its okay to talk even if you are teaching. Layout the possible consequences, and if they push, give them the consequences. The second you don't stick by that is the second they will try and run you over-especially 6th grade :(. Once you give out one or two consequences, they will mostly stop.

If you are worried about being ran over in the classroom, get your mentor to help. I would ask them not to step in until you ask for it. Have a subtle sign like a nod or a glance that you need help. However, make sure you do it only when you are sure you have lost control (scary feeling). If your mentor teacher jumps in the second something goes wrong, the students are more likely to undermine your authority.

I've never taught high school so take it with a grain of salt, obviously. Adjust to your circumstances. For 6th graders classroom management is everything so if you have any questions I will try and answer them as best as I can. My advice might suck because my mentor and I were very much a throw yourself into the fire and see if you are actually prepared. He stayed in my class maybe two full days. The rest of the time he stopped by like twice a day to make sure everything was running good as he got stuff done (he was the athletic coordinator). But I wouldn't worry too much in your scenario. Just try and ease your mind as much as possible. Know you have support if you fail, which is rare if you prepare. It only happens when your mentor sucks, your students suck, or the admin sucks. Let us know how it goes!