Miscellaneous ideas topic.
Inspired by, but otherwise unconnected to, the myth-spell idea:
Paru, pervert that she is, starts another of those 'this will make your breasts bigger' rumour - specifically, rubbing them on Negi's face. Poor Negi, even if he's hit puberty by then, this will still be a problem for him.
Asuna, Ayaka, and/or Nodoka trace the rumour to it's source, and decide that Paru has gone too far, this time, and must be punished.