In my teaching experience with students aged 9-15 there seems to be a good representation of books geared to both genders. But, if you scan through the teen section at your local bookstore (Chapter/Indigo where I live), you'll notice there is a lack of books directed towards teenage boys. The predominant genre is paranormal romance geared towards teenage girls. One only has to see the craze of Twilight everything to notice the popularity of this genre and of course dozens of other series have popped up related to vampires, werewolves, faeries, ghosts, witches, etc...
To prove my point, I looked up bestselling teen books at a variety of popular bookseller sites and discovered that over 75% of the bestselling books are geared towards teenage girls: Amazon: Top 100 Teen Bestsellers, Chapters/Indigo: Top 50 Teen Bestsellers, Borders: Teens Home Page and Barnes & Noble: Teens Home Page
Good news for boys! I stumbled across a website, Guys Lit Wire that is dedicated to bringing books that appeal to teenage boys to light.
This is a site dedicated to books for boys (Grade 3 to 6). Here is a resource list (from 2006) compiled by the University of Northern British Columbia.
Here's another list compiled by the Barrie (in Ontario) public library.
~ a great example of a good book geared towards boys
Another trend is that most stories have caucasian heroines/main characters. Where I teach there is a high East Indian and Asian population. Where are the role models for these demographics that are written in English? How are these girls and boys supposed to identify with characters they can relate to if they are all "white" people? I find a lot of the teens at the school I teach at are reading the Manga graphic novels. Is that the solution? Not sure.