A new yet well-known teen author said she believes teen books should be 200 pages. I have written a teen book of one hundred pages.Let's start here. How are you counting pages? Page count varies greatly depending on the type used, the size of the type, the size of the pages, and the width of the margins, all of which are decided upon by the publisher. This is why most people speak in terms of word count.
It is meant for early teens -- ages 10-14 -- but has all the teen themes, such as first sex, gossip, alcohol consumption, parties.You're writing about sex, booze, and parties but at a middle school reading level? That's... gutsy.
Currently, I have a ten year old and thirteen year old reading my book, which I believe to be the "true test."No, it's not. Whether they like it or hate it, it's not. People who use their own kids as guinea pigs (or other kids they happen to know) are forgetting that the small pool of children available to them are not amalgamations of the tastes, attitudes, and interests of the teen-book-buying public. Individuals are individual. This is a meaningless test.
I understand series type books, such as the Unfabulous series, are considered "teen."By whom? Certainly not the publisher, who lists them as ages 9-12. Don't be fooled by these books being labelled "TEENick"—Nickelodeon knows exactly what it's doing when it packages inoffensive pap for non-teenagers as "teen": they're playing to the wannabe crowd.
Also, unless your manuscript is a derivative of a popular TV show, do not use other TV-inspired books as comparison. Those books are short and meaningless because entertainment companies don't want children to be so busy or absorbed that they forget to watch the next lobotomizing episode.
My question is: Can an early teen book like mine be considered seriously by editors / publishers / agents if it is 100 pages?Ok, for the purposes of this discussion, let's assume you've somehow intuited the page count your book would have if a publisher decided to publish it. The answer is yes. But it's uncommon, so there ought to be something about the manuscript that really makes it shine. Like the writing, for instance.