The author of Wallflower, Stephen Chbosky, has had an interesting trajectory. He went from independent filmmaker to published author to Hollywood screenwriter. While lots of authors become Hollywood sensations because of their books (John Grisham, Stephen King), most of them don't actually dabble in screenwriting. Chbosky carved out a successful career in mainstream film and TV before adapting Wallflower into a film. The book was released in 1999, then a few years later, he was hired to write the film version of Rent (whose characters would mesh nicely with Charlie's circle of friends in Wallflower), then a year after that, he co-created the CBS series Jericho. It probably did help that Chbosky had a background in film and a film agent, but still, it's rare to see a YA author make this kind of transition. Nowadays, Hollywood writers are jumping on the YA bandwagon. (e.g. Paul Weitz, Liz Tigelaar) It can be done, though. In fact, at least three other writers have made the jump from writing YA to writing for film and TV.
1) Rob Thomas
YA books: Slave Day, Doing Time, Satellite Down, Rats Saw God
Rob Thomas has always had a foothold in the teen world. Not to be confused with the lead singer of Matchbox 20, this Rob Thomas went from high school teacher to working at kid's news station Channel One. While there, he began writing Rats Saw God in his spare time, and eventually got an agent. He went on to write five books.
Hollywood connection. On the strength of his YA novels, Thomas was asked to be a staff writer for this new show on the WB called Dawson's Creek. Maybe you've heard of it? From there, he created the series Cupid and then six years later created another show about a teenage sleuth called Veronica Mars. Maybe you've heard of it? Then four years after that, he wrote the script for the reboot of this show called 90210.
2) Jake Coburn
YA books: Prep, Lovesick
Prep, about privileged Upper East Side kids who form violent gangs, caught the eye of MTV films, which optioned the book and hired James Frey to write the screenplay. However, like most book options, nothing has come of it yet. Still, Coburn has managed to carve out a niche in the New York socialites genre.
Hollywood connection. Because of Prep, Coburn was recruited to write a pilot called The Stanton with Kevin Williamson (creator of Dawson's Creek). While that didn't go, he was hired to write for ABC drama Dirty Sexy Money, which revolved around a super rich family in New York City. When that got canceled, he went on to write for a CW drama about privileged Upper East Siders - Gossip Girl - where he's risen to co-producer.
3) Shauna Cross
YA books: Derby Girl
As an aspiring screenwriting in LA with no produced credits, Shauna Cross found a literary agent through a mutual friend who encouraged her to write about her experiences in the roller derby. That turned into the novel Derby Girl.
Hollywood connection. The book caught the eye of Drew Barrymore, who hired Cross to adapt her novel for her directional debut - Whip It, which was released in 2009. Cross next went on to adapt another, very different book - What to Expect when You're Expecting, which was released in June.
One thing to note is that all three writers lived in Los Angeles either when they sold their books or got their film/TV gigs. Unlike publishing, film and TV are industries where newbie writers need to be living in LA in order to be considered for jobs. Sure, if you're best-selling authors like James Frey or Michael Chabon, then you can probably take gigs and stay put where you are.