In fifty words or less, lol?
Wow. This is a huge question and I don't have time in the moment to write a definitive answer. I will work on a hub sometime in the future if no one else takes it on.
Anyone successful at writing for teens must be an excellent writer. Teens see right through any sort of dishonesty, in my experience. Writers can't talk down to them, be judgmental of them, write as if lecturing or attempting to teach a lesson.
Adult writers can stoop to violence or sex if their characters get backed into a corner, but teen fiction writers will not survive if they do too much of this.
Publishers Weekly always has lists of trends in Teen Fiction, like the current and obvious vampires and werewolves skyrocketing to fame alongside the Twilight series.
But I think it is important to write what you know and not attempt to write something that does not come from the heart... that's just me, perhaps.
Teen Fiction follows similar guidelines to any fiction; teens like beginnings, middles and endings. They want fully developed characters and a strong sense of place. They appreciate experimentation and they like to identify with the characters, so teens in their stories need to lead. Often the parents are absent or dead, otherwise they take over the novel, haha.
Oh dear, these are a few thoughts that pop up immediately. I will, as I say, think about it and come back. What do others have to say?