Which word would you take offense to?

I had the pleasure of overhearing a conversation of High School girls here lately. I'll run it down for you and you see where you'd take offense.

They were getting ready for a ball game and some of these girls had changed into their cheerleading uniforms. I refuse to call them skirts because really, they're a wide belt. Anyways, a particular cheerleader had on leg warmers that ran from her heels up to about 3 inches below her uniform. One of the other girls looks at her and says "You look like a slutty Catholic school girl." My wife over hears the comment and says "Ladies, we don't need to use that word." There's a bit of silence and one of the other girls goes "Yea, why'd you have to say she was Catholic school girl?"

I only know of a few types of schools that wear uniforms and honestly the first one that came to mind was the Catholic school. I'm sure there are others but none as prominent. So I guess, if you want to compliment a high school girl, call her a slut. If you really want to tick her off, call her a Catholic.