The big mystery seems to be why enrollments of new kindergarten and pre-K students didn't drop as much as expected. Here is what we know from the very recent 2010 Census of the population by single year of age living in the city of Pittsburgh. So children who were age 3 two years aro are now 5 or so and in those pre-K years.
Sure seems to me that the birth years coming into the school system are trending up. Population in the city under age 1 in 2000 (so born between 4/1/2009 and 4/1/2010 techically) is 28% higher than the population born 6 years earlier and still residing in the city. Now it is not that simple and for school districts there is a lot of inter and intra-regional migration impacting enrollment trends.. something that is surely true for the city's school system. It might be that young families are more likely to move out after a child is born which might produce that pattern, yet not produce increasing enrollments by the time the children reach school-age.
So let's compare to 2000 at least to see if the pattern is really just a repeat. Here is the same data from both 2000 and 2010.
The 2000 data does not show a decline in population by birth cohort leading into the earliest school age. So at least superficially there are a couple things to think about. Looks like right now there are more kids heading into the schools within the city. Why? If you infer from the 2010 data there are some bigger cohorts coming; those who were age 1 or 2 in 2010 are bigger than the cohorts just older than them. Lest that be taken a bit too optimisitically for the school district look again at the comparision to 2000 and note that the cohort sizes for every year are far below where they were a decade ago; those under 6 or 7 in 2000 likely are just graduating out of secondary school right around now. All recent age cohorts are smaller than any cohort a decade earlier. So the long run steady state for the school system may still be trending down a bit as these smaller and younger cohorts age through the system. Still what deserves more thought is just how different the two years look. The 2000 data actually looks like it had bigger age cohorts as you get older, the opposite pattern of 2010.
But for those who want to look more at trends in City of Pittsburgh demographic trend we compiled that all to death a year ago. The truth is there are a lot of differnet things going on. Different migration patterns for different types of households and and far different trends in different parts of the city as well. So no one story.