some are easy. A la Harry Potter's Christmas stays. But some are more difficult -- rather like Ron's staying, or Hermione's, where the reasoning seemed rather weak. Loving families will want to see the kid again, and the kid won't actually love staying at school however fond he is of his schoolmates.
On the other hand, I think this culture has more tolerance for long trips. The travel magic is not quite up to what we consider normal.
And I have to decide when Susan's parents will vanish. That's kind of important. Parking her with relatives and then letting her stay at school is one thing; never seeing her again is another. Timing, however, is crucial, since it will trigger other events.
Meanwhile, I suspect that some of them have to go home, some of the time.