PM
Take a look at the definition of centrist and tell me how many you think picked up one day and started to go to yeshivish schools – if anything they would go to MO schools. You do however bring up a valid point – maybe part of the growth is due to immigration not natrual growth. This is true when comparing overall two population datasets taken at different times (such as comparing the two studies). When the growth rate is derived from the ratio of younger to older members the reason for the growth rate becomes irrelevant. The important point is that there are more 10th graders than 12th graders.
If more chools where discoved by more experrienced survey there would consistently be more 6th, 7th, and 8th graders in the second study than there are 1st 2nd 3rd graders in the first (as they are the same class) this is not the case (at leasr with regard to yeshivish)
I do not have any evidence that smaller schools were missed I just mention it as a possibility that seemed to make sense to me.