There's nothing, really, that we need to remember when writing for teens. Absolutely anything goes.
Ambiguous endings, possibly. As in, avoid them. I recently finished reading an adult novel which ended that way, and forgave it (in spite of my own frustration) because the author was making subtle points about fiction, war and humanity. Having said that, I'd be similarly forgiving with a YA novel, if it were well-written.
But, yeah. I probably would think hard before writing an ambiguous ending to a YA novel.