Dancers in ballet classes should have their hair in a bun on the back of their head, secured so that it stays in place. The following video tutorial gives you an idea of how to make a bun. Placement should be to where the ponytail is even with the dancers eyes.
Please do not send your dancer to class without their hair pulled back! It is distracting to the dancer and leaves the teacher or the office staff to do it, and could incur a charge to your account.