Have you looked up the reading levels of the books he reads at home? Does he read a lot and with comprehension? With a WISC score like that I'd assume his verbal score is very high, but is there any chance he's not too much of a reader and is more quantitative? I would still guess that "on grade level" is wrong. They may just not be paying attention. Sometimes schools say "on grade level" and what they really mean is "we perceive no problems here."