As an instructor, I am very clear that if you come to a level 1 (i.e. beginners) class you are working on beginner material. The same holds for level 2, 3... I do not want students distracted or intimidated by what their classmates are working on, nor do I want "adventurous" beginners trying things they have no business trying because they saw someone else do it. I do tolerate students doing the same move that is being worked on, but a slightly more difficult variation--if we're working on fireman spins, and you can safely do a one handed version, fine, if you do a obviously different, showy spin (e.g. reverse grab) or start inverting, you'll be politely asked not to do that right now. I am clear and consistent about this policy, and for the most part, students respect it.
I don't regularly take classes, but I do visit other studios whenever I travel (which is fairly often). I typically sign up for classes that based on description are below my ability level (a) because I'm often tired on work trips, and (b) to make sure I'm not signing up for a class that is above my ability level. If I show up and we're working on the fireman, that's what I work on. Occasionally I'm asked if I've done this before and I'll admit I've been pole dancing a while, but unless the instructor encourages students to work on more advanced material, I just enjoy the class. I often find I learn a new transition, or slightly different way of doing something, things I might miss if I tuned out the instructor because I have already mastered the move being taught.
A couple of other things:
I have split classes into beginners and not beginners before out of necessity. I found those classes are very taxing to teach, and I'm not sure my students got as much out of them. If you're going to offer what is essentially an open/mixed level class, why not just call it that, at least students will know what they're getting.
Personally, I also avoid letting students act as "junior instructors." There are some fairly serious liability issues. Also, just because a student can do a move, does not mean they are going to explain how to do it well or provide useful/safe advice to other students. An occasional tip is fine, but if I see much beyond that, I'll pull the student aside and politely ask them to stop.