You need to ask what test they are using, how the test is administered (how are the questions asked, how is the student asked to respond (verbally etc), is there a time limit on answers, when is the test stopped - ie is the full test given no matter what or is it stopped after a certain number of questions are missed) and what specific skills it is testing. Also ask about *when* the test was administered and who administered it - was it a part of an individual evaluation and administered by someone from the gifted or sped department, or was it administered as a routine part of class testing where the teacher was administering it to the full class? You'll also want to know which day it was given to your child, in case it fell on a day when he'd been up until midnight or the day before he was out of school with the flu etc.
If you know the name of the test and can post it here, that will help us with giving suggestions to.
For instance, the DRA is stopped after a certain number of questions aren't answered correctly, and it can also be stopped arbitrarily - this happened quite a bit with my ds in early elementary; he'd get everything 100% correct up to grade level and the teacher would stop there and move on to other students because the testing was taking place during class and the teachers needed only to catch the students who were struggling and didn't have the extra time to administer the test fully to students who were reading years beyond grade level.
If it's a test like DIBELS (which is used routinely as a screener in our school district), and your ds has uneven scores on the subtests, it's possible your child actually has a reading challenge that has gone unnoticed (it really is possible for kids with extremely high IQs to have reading challenges).
If it's an AR test.... they don't test *much* - the child just answers a set of something like 10 questions that are meant to test comprehension. It's always possible that a student might rush through the test simply because they aren't invested in worrying about getting the answers correctly or they didn't read the book they were required to read carefully simply because it was way below their comprehension ability and way below the books they are interested in and read at home. So if it's an AR test, I'd ask for him to be tested on a book that he's read at home and that you feel is reflective of his true reading ability.
Last thought, once you have the test and if the school is still insisting it's accurate, I'd make a formal written request for more in-depth testing on reading skills because of the discrepancy between his reading level (per the school) and his ability level (per the WISC). I seriously doubt your ds has a reading challenge... but that additional testing should make it clear that he doesn't

And... last bit of advice... I'd put together a list of the books he's read at home in the past 3 months and have that in my back-pocket when meeting with the school.
Best wishes,
polarbear