killerbee did not use an unforgivable curse. She did use the similar sounding "adava kedavra" curse, which has once been invented by Filligus Minneva, a very well-known Gryffindor, to scare fellow students, but do no harm.
I suggest you teacher look it up in "A History of Funny Curses" or "Hogwarts: History of Curses".
Here is a quote from the latter:
Filligus Minneva invented the Adava Kedavra course to impress and scare his fellow students. One night, while taking out his prospective girlfriend, he pointed a want at someone who entered the secret room in which they had spent some time. But instead of using the unforgivable curse on him, the room filled with flowers. Needless to say, the girl has been his girlfriend then, they even got married after they left Hogwarts. The curse cannot even be called a proper curse, but it is, of course, a harmless spell.
There you go. I just made myself headgirl and I am demanding the points back for Slytherin. I will take this to the board if need be.
Killerbee has been out of hand a bit, and you could possibly deduct 5 points from the house because of her, but you should not make the house suffer under false pretence. Darth_vader is ok, he has just been too lazy to come out again. After all, noone knew he had a flower allergy when killerbee said the spell.
Even though us Slytherins seem to be thought of as bad, we are not all dark wizards and witches. You, as teachers, should know that.
I also demand that our head of house is around more often!
As for my fellow students, it is ok to have fun, but please, don't get us in trouble anymore
Do that stuff in (semi) private!