Do you have a copy of the WJ-III test scores, including subtest scores? If you don't, the first thing I'd recommend is that you request the full report from the school. once you have the list of subtest scores you can look to see if e scores at consistent or if there is scatter, which can happen for all sorts of reasons. Most of the WJ-III Achievement subtests aren't timed but there are a few that are - they are labelled "fluency" - and if your ds seems to have challenges with timed tests in class that might be what brought his scores down. If you see names of tests that have the word "Broad"in them, they are the average of subtest scores in a category, for example "Broad Reading" averages together the three reading-related subtests.... so you might see something happen where untimed test scores are high but a low tx test score can bring down the average that goes into the Broad score.
I'd recommend more testing -part of the reason I recommend it is that I have two 2e kids, both of whom looked clearly gifted to the world before they started school, did ok through K-1, and then their worlds fell apart in 2nd grade when they could no longer compensate. We had no idea they had any kind of challenge until they we tested. For my ds, one clear sign we saw in hindsight was low performance on timed math facts tests. Chances are your ds doesn't have any type of challenge, but if he does, you'll be so glad you found out now instead of further down the road... plus you'll have data to use when advocating for gifted services, and hopefully a better understanding of the school's test results.
Best wishes,
polarbear
