1. NO TALKING!!!!! Talking is for before and after ballet classes, not during the combination. The only thing you should say, is "FIRE!" if you have to =)
2. Don't correct other people. Don't say "Psss. Lift your arm higher!". 2 things: 1) It's the teacher's job to correct, and 2) You might be lifting your own arm too high!
3. Hold your finishing position until the teacher says to relax. Even though the music has stopped, your teacher might want to walk around and inspect everyone's finishing position. This especially applies to barre work.
4. If you are in line waiting for your turn to do a combination, don't skip ahead and cut because you know and love the combination. It is respectful to stay in line where you were the entire class.
5. Don't ask questions in the middle of class. Come to the teacher after class and ask about the combination or step.
6. If you have to leave early in class, 1) inform your teacher at the beginning of class, and 2) don't ask him/her what time it is. It's impolite.
7. If you won't be there the next class or so, tell the teacher that you won't be there. Just this week, someone didn't tell my teach that they wouldn't be there, and we had to start class late.
8. Turn your cellphone off. Most of your friends know not to call you at a certain time, but I have gotten wrong numbers and service calls during class because I didn't turn my phone off.
9. Listen to the teacher. If you don't know if you heard 6 or 8 frappes, too bad. It's not polite to make the teacher repeat it.
10. Don't complain or groan. Believe me, he/she will pile more work on your plate.
It's only fair for me to say that I have learned all of this from my own previous experience ;)