I'm trying to find a story I read awhile back. A 9th grade boy, Butch, is flying to spend the summer with his best friend while his Mom is away working an undercover narcotics operation. He arrives, but his luggage doesn't. His best friend dares him to wear little sister's (Zoe?) one piece bathing suit so they can swim in the backyard pool. While wearing it he is seen by Zoe and her friends, who assume he is a 7th grade girl like them. He plays along and gives them his given name, Stacy. Stacy is taken into the girl's group and ends up spending most of the summer with them and having a great time. Then after Zoe discovers Stacy's secret and threatens to expose Stacy as a fake, Stacy runs away and ends up being physically assulted. Stacy wakes up in the hospital suffering memory loss, thinking he really is the girl he has portrayed and remembering nothing of his boyhood past.

Dang, I can remember way too much of the plot, but only a few names, none of which are the author's name or the story title, and a search has turned up nothing. Can anybody help?