I'd never heard of the Kill/Boff/Marry guy rating system until I read this post on Forever Young Adult regarding The Hunger Games . (You should read it. It's hysterical.) I guess I was ahead of my time because in high school, I had my own guy rating system. And this was before Twitter, Facebook and when typing in shorthand was just catching on. (My favorite: a/s/l)
Working at my first job making pizzas, one of my female coworkers and I needed a way to rate the male customers who came into the store without them knowing. This spawned B.I.G.
I: I'd do him (except we didn't say "do")
Then we had variations because not every boy fit into one of the above categories.
BBI: Beast but I'd still do him (for those nice/fun guys who aren't an A+ in the looks dept.)
GG: Gorgeous God
GBB: Gorgeous but beastly (for all those good-looking jerks)
The slang may have changed, but teenage girls have not. How many YA protaganists are 16-year-old girls? A lot. And this is what teenage girls do. Think about boys, talk about boys, whisper about boys. They share private jokes with their best friends and sometimes even make-up their own language.
Of course, teenage girls aren't one dimensional. When they're not thinking about boys, they're worrying about their appearance, dealing with their parents, planning for the future, passing their finals, picking colleges, standing up to bullies...and on and on and on. I think that's why so many writers like to write YA. There's never a shortage of drama.
But adding little tidbits to your novel like a secret guy rating code between friends adds realism to your characters, because it is real. I couldn't have been the only teenager to do it. Keep that in mind when crafting your characters. The quirks and details are what turn a series of words into a real person.