Parenting 101

You are the professor, and I am the student. I have too many children, and often I come across an issue that I simply don't know how to handle. So I would love to know how you might handle it if you were in my situation. Maybe offer me a much calmer alternative to what I would instinctively do. Because I may only be half Greek, but I have a full Greek temper, and it's not good.
So tonight's lesson will be: cussing.
My dear, sweet, innocent Allison, yes the one you all LOVE, had some friends over today. They spent a good majority of the evening walking around as if they owned the Sac, teasing younger children (mostly her sisters), playing Truth or Dare, and God knows what else those preteens do.
Well a little boy who does not frequent our Sac, came over to play with his friend. He found a volleyball and asked Alli if it was hers to which she replied," Who the hell are you?" Yes, this is the Allison that you all LOVE. We were in a public setting so beating my child in front of my neighbors and her friends did not seem a viable solution. Honestly it is the first time I have heard her speak that way. And even more honestly her mother does have the mouth of a sailor.
Later on she apologized again for using those words, but said that EVERYBODY at school talks that way, so it just slipped out. Does that mean she speaks that way too? And if she does, how would I know? I can't punish her for something I don't know about. Was she just trying to be cool in front of her friends? Is she too old for me to wash her mouth out with soap? Suggestions?
It's coming: puberty, adolescence, attitude. And I'm scared, very scared. Those of you who have requested Alli in the past, if you still want her, I'm taking bids.