Just now reading the replies to this (sorry!)...
I'll just leave it alone. We're already afterschooling DS 2-3 grades ahead at home, so I'm not sure what I was trying to accomplish. Seems like a total waste to just watch the kids sit there bored, but I don't want to risk upsetting the teacher/school by doing too much.
Sorry if I sound defeated.
This whole interacting with the school thing feels more complicated than it should be.
I did successfully teach a lesson for Hour of Code on coding to the class, during math time, which was designed to naturally allow the advanced kids to move at their own pace. It was a huge success and the teacher asked if I could come back and do another activity. So at least there is some minor success story to report there! I got to teach recursion to a K student that is always way ahead in math -- that was awesome.