Bronxalea - I think you're asking the right questions and doing a good job trying to assess what's what, which is easier said than done.
If the school isn't challenging or offering enough for your ds in terms of math, then that's likely a concern and a source for behavioral issues. Of course, that's only one subject too. You'd have to take a methodical look on behavioral and the points where school isn't challenging your son.
The other aspect is that IF you address/eliminate the sensory issues as much as possible and then the behaviors continue to occur, then you'll know it's not necessarily a sensory issue and more something behavioral.
Another suggestion - how much can you supplement at home or how much can your child bring supplemental material to school and do? Perhaps it's possible for your son to quietly have something to do when he's feeling restless? Is he allowed to get up and take a walk/go to the bathroom? Or is this not an option? I ask because my son was at two gifted schools where it was possible to get up and move around when he felt restless and fidgety and this really helped. Of course, it didn't address the lack of not being challenged but at least I could make a better assessment/decision based on knowing my son could get up and walk around when necessarily. I know there's only so much you can do - but I think the trick is knowing what's in his control, what's in the school's control, and what's in your control so you can get a grip on what's going on and find the holes with your son.
VT - it can take some time before you see improvements. When ds8 was in vt between 4-6 yrs old, I remember it took about 6-8 months before we saw the first wave of big developments and changes. In Dec, ds started to wear glasses again and we haven't noticed any dramatic differences yet either, but I know these things often take time.
