Teen novel, must it be first person?

No, you are far from bound to first person. Afterall, look at Harry Potter, it's third person. There's an abundance of both first and third person in teen books.

Write it however you feel most comfortable. First/Third...doesn't matter. Great writing will get their attention.

I think it depends a lot on what you want the book to be about. Robert Cormier, who was an extremely prolific YA writer, wrote almost exclusively in third-person. In all his books, he seems mostly interested in exploring the psychology of particular characters, or the way different characters all react to the same situation. The only instance I can think of where he used first-person was in Tenderness, when he wanted to show what a distorted, juvenile view of reality the MC was functioning on.

To me, first person seems most useful when you think the manuscript should have a “confessional” feel to it. Adam Rapp, Laurie Halse Anderson, and E.R Frank all write wonderful YA books in first-person. The books work really well because they are always about how one very specific person sees the world. So, I guess what I’m saying is, if you feel like your book is about a particular idea, the easiest way to express it is probably third-person (notable exceptions being To Kill a Mockingbird and The Catcher in the Rye). But if you’re in love with a character and you think everyone else will be too, that’s when first-person is at its best advantage.

Harry Potter
Artemis Fowl
A Series of Unfortunate Events

....perish the thought that you have to write in first-person. Actually, I think first-person writing is a much-maligned thing that is frequently used as a cover for problems with the main character, or the story itself. I think it's too bad, because done well (I, Jedi - The Chronicles of Amber - The Icarus Hunt) first person is a delight and a comfortable thing to read.

For the life of me, I wish someone would have a definitive answer.

There aren't a great many definitive answers in writing. Just good ideas, and bad ideas.

you're fine with third person in a YA. I could add to the already-begun list, but really there's no need. If you look at the YA bookshelf of your local big bookstore, you'll probably find as many novels in third person as you do in first.

First Person POV isn't required unless the publisher's guidelines say "Must be First Person."

What is required, always, is a compelling story compellingly told.

First person not required. I have two teen books out (well, one forthcoming); first was strictly close 3rd person pov; 2nd has two protagonists who alternate with 3rd person pov chapters (which sort of breaks a rule in itself, having two protags).

The one that I'm writing now is first person, and this is because the "I" guy is, for the most part, an observer relating about the real central character.

But James Macdonald (being new to this board, I'm starting to figure out who he is) says it best with fewer words

Elise_of_the_Isle said:

Most teen books have the blank main character who is overall pretty dull, but they toss in the first person.

Click to expand...

To be quite frank, I question whether you should be writing YA at all with an attitude like that. At best it shows that you have not bothered to read widely in this genre - otherwise you would have come into contact with a multitude of complex, vivid and memorable YA characters, both first person and third person. At worst it betrays a contempt for the genre, and the idea (often mooted by celebrity authors) that 'there aren't any good books for young adults so I'll write one'. Wrong, wrong, wrong.

The quote does sound like a (slight) bit of contempt for a mainstay of the genre, the (as you said) Blank Dull Main Character...but consider this...for some YA writers, this is an intentional device. So-called "blank dull character" evolves, grows, gains new experiences, develops insight and typically comes to some sort of "coming of age" event that builds the character - right before the readers' very eyes. It's a common device, and first person is a clean way to do it.
Quite simple, really, and since teens are often cumulatively at this level of life, it is a "marketable" strategy, right?