Their ability to correct for the missing kids, is presumably because they can assume that the kids missing are missing at random (proportional to their attendan

Their ability to correct for the missing kids, is presumably because they can assume that the kids missing are missing at random (proportional to their attendance rate.) This is very different, because while there is some data missing, it is missing at random (albeit weighted), and can thus be corrected for. If it was the case that kids avoided coming into school that day, because they knew the survey would be held, it would be a very different problem.