Relieve or Backstabbing?

Hey guys, first post here. (pls bare with it) XD

Well, this happened in my school, one rainy morning:
During assembly time, a group of girls were gossiping, I (a prefect) went towards them and advised them to be quiet (XD I'm a responsible guy!), but to no avail, they kept on with their conversation. The principal who was "lecturing" on the stage ask two of the girls up, and so did they. Then, one of the girl came to me ( I was beside her) and said: "Luckily it wasn't me! "  while soothing her chest with relieve.

So, maybe you guys might feel what's so big deal about this? Is is supposed to be sarcastic?
Well, for me, friends should help each other, and in this case, I call it "backstabbing".
Call me sensitive, and you will NEVER understand how the girl felt being embarrassed on stage, and worse,
 if she finds out about that.