When my oldest DS was in preschool, he had trouble sitting still during circle time. He's always been very tactile and very observant and would instantly notice anything new in the classroom and want to go and check it out. Turns out, the teacher had been intentionally bring new things into the classroom on a regular basis to make it more interesting for the kids. But, since every time she brought in something new she noticed tht DS would be drawn to it, she significantly cut back on the frequency of adding new things (brought in the same number of new things, just more at one time) and allowed the kids extra free time on those days to explore the new stuff. Another thing that she found that helped DS sit still during story time (when they were reading things years below what we were reading to DS at home) was to have one of those squeezy-ball things that he could just hold and quietly play with while sitting on the rug. He's 12 now and, while he's still very observant and tactile, he's probably one of the calmest kids in his classroom. Side note: he was eventually "diagnosed" as ADD, but inattentive -- definitely not hyperactive.
DS6 was in 1st last year (skipped K) and for the most part did quite well sitting and working, but during story time on the rug (again, reading books we had read years before) would move around quite a bit in his spot. The teacher's solution was simply to place him in the back of the group where, while she would encourage him to sit on his pockets, he didn't disturb anyone. Likewise, since DS sometimes likes to stand up at his desk while he's doing his work (but doesn't wander) he's just placed toward the back so he doesn't block anyone's view. Interestingly, when he's in 3rd grade math, he has absolutely no problem sitting still while he's working and his 3rd grade math teacher often comments on his maturity, while his 2nd grade teacher says he still likes to stand. So, challenge may definitely be a factor.
Honestly, try not to worry too much. Sometimes early education teachers have the tendency to pathologize totally normal boy (especially) behavior. Unless you have a gut feeling that something isn't quite right, don't let the teacher convince you there's something "wrong."
