Okay, more recommendations.
Rainbow Boys (2001): Nelson, Kyle and Jason all go to the same high school in Maryland, where Nelson is the only openly gay kid in the school; Kyle is on the swim team and closeted to everyone but Nelson; and Jason, a basketball player with a pretty girlfriend, wonders why he keeps thinking about guys. When Jason forces himself to walk into a meeting for gay youth, Kyle and Nelson see him there, and things will never be the same again...
I read this book an insane number of times. It has a nice portrayal of bisexuality - Jason is truly attracted to Debra, his girlfriend - and a really sweet romance that grows between Jason and Kyle after Jason and Debra break up. Nelson has to deal with his unrequited crush on Kyle, his best friend, and watching Kyle be happy with Jason. There are also plotlines about body image, homophobic bullies, and why it is a terrible idea to have unprotected sex with a random guy you meet online. (Someone what sexually explicit, but not overly so.) There are two sequels, Rainbow High and Rainbow Road (the one with BJ I mentioned before); this is definitely the best one because it feels the most natural. The later books give the impression that Alex Sanchez, the author, is trying to take the opportunity to mention every single LGBT issue in the world, and it feels forced.
Peter (1991): This is out of print, but I thought I'd mention it quickly anyway. Peter is a teenage Australian kid who meets David, his brother Vince's friend, and is intrigued because he's never met anyone openly gay before. He wonders about his own sexuality and eventually makes a pass at David, who turns him down and says he's too young. The book doesn't feel the need to have Peter declare an orientation by the end, so it's a good one about exploring sexual feelings without having to label oneself.
Am I Blue?: Coming Out from the Silence (1995): Oh God, I love this book so much. It's a series of LGB-themed short stories that run the gamut from fantasy to historical fiction to contemporary fiction. We've got sexual identity crises in the 1950s, gay Vietnamese soldiers, a lesbian who wants to out herself to her grandmother who survived the Holocaust, a teenage boy visiting his dad after his dad's male partner dies unexpectedly, and a genie. Yes, a genie.
Maurice (1971): It isn't actually a YA novel, but it deals with the title character's life from 14 to 24. This one is for the highly literate history and literature nerds who love period romances and stuff like Merchant Ivory. In fact, there's a Merchant Ivory film based on this book.
Maurice Hall is a middle-class, eminently average English boy living in the 1900s and 1910s. He goes to Cambridge and meets the clever, someone neurotic Clive Durham; a romance develops, but Clive is insistent that it be "purely platonic," with no sex and the occasional kiss. Maurice is in love with Clive and willing to build his life around their being together, but a couple years after university, Clive insists that he likes women now and wants to get married. And he still wants to be friends with Maurice, despite the obvious fact that it's crushing Maurice's soul to be around Clive and his new wife.
Then, plot twist! While Maurice is visiting Clive at Clive's family's country house, he's seduced by Clive's under-gamekeeper, Alec Scudder, who is affectionate, carnal, emotionally open, and everything else Clive is not. He's also moving to Argentina in a few weeks. Does Maurice have a chance at a sexually satisfying love, and if so, will he take it?
E. M. Forster, the author, didn't want this book published until after his death, and didn't see it take on classic status. The 1987 movie is just fantastic. A very young Hugh Grant plays Clive, James Wilby plays Maurice, and a baby-faced Rupert Graves plays Alec Scudder. The book is very fade-to-black on the sex scenes, while the film has full-frontal nudity, so that's something to keep in mind if you're going to recommend it to teenagers.