I would not be at all concerned that 6th grade state score was higher than 7th if it were either of my DS, particularly if he's still at a 95. Our state scores can drop by 5% if they miss one question (this is because the scores are reported as percentiles--your state might be different). As you said, depending on the day he might have a different score. Both of my DS math scores on state tests have fluctuated over the years.
I would be concerned that he finished all of the material in 7th grade advanced math by January and the teacher wouldn't give him anything else to do. Does the school already know about this situation? If not, maybe address that with the administration before the next school year and ask what they have in place for next year for students who work at a faster pace. Maybe one teacher is more appropriate than another.
When dealing with the public schools and their limitations as far as budget and curriculum are concerned, the right teacher can make a big difference.
