Arrive early to setup the piano/space. If you are going to be late, be sure to let the teacher know.
Arrive with a good attitude. Be sure to go into ballet class with the idea of open communication. Do not be afraid to ask questions of the teacher when something is not clear: meter, tempo, accents, etc.
I recommend for new accompanists to meet with the teacher outside of class to discuss things.
If you are not sure if the music you have picked out for the combination is appropriate, ask to play it for the teacher after they are done demonstrating the combination to the dancers.
It is integral that the pianist has a system by which they organize their music for the ballet technique class.
The pace of the class is generally fast and pianists should be prepared to play the music immediately after the teacher has taught the sequence of the steps in each combination (generally they will give you a cue like a smile or say “and”).
There are different ways to organize your music: by key signature (2/4, 3/4, 4/4, 6/8, etc.) and by movement (plié, tendu, dégagé, etc.).
The latter is what I recommend for the beginning ballet accompanist.
Do what makes it easiest for you to find the music you need.
Do not eat during class.
Do not leave early unless there is an emergency or it is prearranged with teacher.
Avoid making faces in reaction to what the dancers are doing.
For more recommendations of things to do or not do, see Cavalli (2001) pp. 95-96.