Sometimes dialogue in Y.A. fiction can be over the top. "So, like, I was talking to Jon the other day, and he, like, asked about you." (This may be a slight exaggeration.) But some use the argument that if it's a teenager talking, hey, it should actually sound like a teenager, right?
Sure, realism is great. But I think that when it comes to young adult novels - any novel, really - there's a certain way to do things. We have the opportunity to create some meaningful conversations and weave a great story. Do we really want to ruin it with annoying, grating, insulting dialogue?
Be real, but not too real.
If that makes sense. Throw in a like now and then if you really want the teenager vibe to roll off the page. If your character isn't the kind of person to use those long, descriptive words, don't use them. But don't deliberately dumb everything down.