What's the Name of That Book??? discussion

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Probably Still Nick Swansen
SOLVED: Children's/YA
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SOLVED. MG/YA, read in ~2001, told from the perspective of a character with an intellectual disability; pivotal scene involves sister and pool; big part of the book is about narrator's experience in Special Ed. Spoiler. [s]
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I remember one major plot point: the narrator's sister(?), who was not disabled, (view spoiler)[drowned in a backyard pool, maybe while showing off; the narrator had a sense of guilt for not realizing that something was wrong and help was needed. There may have been a sense that the adults around held the narrator partially responsible as well, and that they held a preference for the non-disabled sister (hide spoiler)]. I think this happens early on in the book.
A fair amount of the book takes place in a Special Ed classroom, and there's one girl in the class who everyone thinks is super smart and competent and pretty; later in the book, she's mainstreamed, and the narrator has a moment of realisation that although this other character was the star pupil in Special Ed, she was treated as dumb and unimportant in the wider school. I think there was some level of plot or subplot about dating.
I thought this might be something by Cynthia Voigt or Virginia Euwer Wolff but haven't found anything to match in their backlists. It was a standalone, or if there were sequels I was not aware of them at the time. It was probably a mass-market paperback, and I'd be surprised if it was more than 200 pages long. I don't remember anything about the cover.
Thanks!