This was suggested by Liza E. way back in January: Marble Ace Arvidson, 17, missing from Brattleboro, Vermont on August 27, 2011.
Marble was classified as a special needs child, but I’m not exactly sure what his special needs were. He was in foster care, but it doesn’t seem to have been a “typical” foster home: this article says, “Marble lived with a mentor — a legal guardian in his 20s — along with another teenager and that teen’s mentor.” He had some emotional problems and issues with aggression, but he was considered high-functioning and planned to attend college. One of the schools he was looking at was the University of Vermont, and I know you need to be a good student with above-average test scores to get in there.
His case is a strange one because he appears to have simply dropped off the face of the earth. There’s some suggestion that he might have drowned in Hurricane Irene, which was causing raging floods in the area at the time, and that would certainly explain things, but there’s no actual evidence to support this theory as far as I know.