No matter the level of a dancer, there is no excuse for talking during a class or rehearsal. It is entirely disrespectful to the teacher and to fellow dancers. The worst offense is talking while the instructor or choreographer is also talking, that is not only impolite in general, but beyond rude. This person is here, dedicating their time and effort, to make you a better dancer. Abstain from talking during class as much as possible and show your teacher that you are focused and eager to learn. It goes a long way.
A dancer should not wear clothes that will distract themselves or others from their main focus. The proper dance gear should be worn per dance style. Ask the teacher for the appropriate type of attire.
Make certain that you are friendly with ALL the other dancers in the classroom. Make an effort to make people feel welcome and comfortable. Too often dancers get arrogant and they don't associate with other students who aren't on their "level". That shows immaturity and lack of compassion. It is hard enough to learn as a newcomer, so be kind and open to your fellow dancers.
There are always those handful of dancers that arrive late to a class or a rehearsal or miss often. When a dancer comes in late to class it not only affects the focus of the entire class and the teacher, but it is very rude, disruptive and disrespectful to everyone involved. Coming in late to class usually isn't allowed since lacking a proper warm-up could cause serious injury.
Every so often a classroom will be a bit crowded. Students need to be extra careful in these situations to allow even and proper spacing so that all the students can get maximum visibility and use out of the class. It is uncouth for a dancer to step directly in front of another dancer who was already spaced and there before them. Certain dancers choose the front of the class, that is fine, but if you aren't already placed in your "spot" before the rest of the class settles in theirs, don't walk in front of the others and block their view. It isn't thoughtful or kind.
The worst offense a dance student can do above all the others, is to disrespect the teacher. Disrespect comes in many forms. The above mentioned are some of them. When a teacher is talking, you should be listening quietly with a perky ear. Think of their lesson as an investment to your dancing education, be thankful for it and show courteous behavior. Each time a teacher says a correction out loud, assume it is you and check to see if you need the same adjustment.