An explanation of enrolment projections

Given the comment on the previous post, I had been holding this tab open in my browser for almost a week now from the Welland Tribune. The article concentrate's on a District School Board of Niagara superintendent's explanation of enrolment forecasts at one school that changed dramatically from one year to the next. In a coincidental twist, I got an e-mail months ago on DSBN and enrolment forecasting, after I posted something on the board's decision to hire a private consultant, Watson & Associates Economists Ltd. Apparently there's been some shift between people at the private firm and in the board's planning office, and the thought was this was contributing to a, er, different take on the enrolment projections for the schools involved in this Welland review.
From the article:

The projected enrolment data originally presented to the assigned committee last October, when the review began, placed Crowland's capacity at 78% by 2018.
Updated information was provided in December, placing the school's capacity at 50% by 2018.
Parents questioned why there was such a big change in numbers, a question Kartasinski hoped to clarify at Tuesday's meeting.
...
(Linda) Kartasinksi explained that enrolment numbers are projected using a formulaic process.
It begins with "straight-line enrolment," taking the current number of students in each grade at a school and moving the numbers forward a grade. Using birthrates within the school boundaries, staff in the planning department work to predict the number of junior kindergarten students that will be entering the school the following year. They also take into account the number of students graduating, she said.
For instance, Crowland Central was predicted to have four junior kindergarten students next year, with 28 Grade 8 students graduating.
"You move 28 out and four up. That's a net loss of 24 students," Kartasinski said. 

What's missing here that many school boards also use is any impact of redevelopment or development of new housing in the school's boundary area. A birth-rate formula is also applied to those housing units (per year, based on projected build-out) to estimate how many children will live in the area.
To all these formulas, the boards must also consider the traditional percentage-split for how many children go to English public, English Catholic, private, French-language, etc. They still can't count on every child in that housing going to the local public school.
And yes, enrolment is continually revised. The reality really is in many, many schools there are far larger cohorts of students leaving elementary schools from Grade 8 and moving onto high school than there are kindergarten or Grade 1 students coming in the other end. This rationale doesn't preclude blips in enrolment (you know, something in the water...) which still can and do happen. However a blip is not a medium- to long-term enrolment trend.
While we do focus declining enrolment so much on the elementary schools, it's about to hit high schools hard and fast-- a decline that takes eight to 10 years working through an elementary school moves through a high school in four to five years. The largest grade cohort in my region is Grade 11-- meaning the numbers of students in every grade below Grade 11 are smaller than the ones above.