I think that they were, like in the manga, students in Mugen Academy. For Viluy and Tellu, it was official, but I'm sure it applies to Mimete, Eudial, and Cyprine as well.
They are probably older students though. Maybe from the college section...?
No wait, the ribbon on Viluy's uniform was for the high school... She would have to have been a thrid year then.
Maybe Eudial, Mimete and Cyprine already graduated? Nothing says the Witches are all the same age.