depends on what kind of school it is and on why they need a pianist:
1. the dancers learn new variations? then you will be given scores for those variations and everything is pretty easy - just play the score.
2. the dancers learn classic ballet, you are needed to play music for their exercises. now, you probably could find/ask the ballet teacher for scores for each type of exercise (and believe me, there are many types - depending on how advanced the students are), or - and if you can do this, then you have no problem - you can ask the teacher what kind of music s/he needs for each type (what kind of beat, rhythm, auftaktic or not, etc.), write this down, and then improvise every time. if you choose to improvise, it's also good because you won't repeat yourself and it won't be boring. and you don't have to improvise complicated stuff because they need the music to be simple.
best luck