1. Seat CushionA semi-inflated rubber cushion with bumps for lots of sensory input can literally give your child some wiggle room -- he can get the feeling of movement without making too much of it. The circular Disc O' Sit, in 12" or 14" diameters, is one to try.
3. FootrestOften, kids whose feet don't reach the floor feel unsettled and unbalanced, and are more likely to kick, fidget, rock and roll in their seats as a result. You don't need to buy a fancy footrest; just place a brick, block, box or other hard heavy item beneath your child's feet so that his feet rest firmly on it and his legs are bent at a comfortable angle. Make sure the item's heavy enough that he won't be pushing or kicking it around.
4. Separate Desk and Chair
All-in-one desk-and-chair combos are dangerous for active kids, since rocking the chair means rocking the whole desk and sometimes knocking things off it. Your child may be more comfortable in a desk with a separate chair; if the teacher can find one, it may make a real difference in classroom decorum. (Slipping a cut-open tennis ball at the end of each chair leg takes the noise out of any scooching and sliding that does happen.)