Just as overall population growth is unevenly distributed around the region, so is growth in school enrollments. And the two are not as closely related as you might think.
For every 100 new residents of the three county region in the past five years, about ten new students enrolled in public schools. Nationally, there are about 15 public school students for every 100 Americans, so the Puget Sound region is pretty short on students among newcomers. But as the Indexer has noted, the Seattle region has been increasing its population of young, childless adults at a furious pace, so a lack of students is not entirely surprising.
Figure 1 shows the growth in student enrollments in the regular public school districts of the three county region. Tribal schools, charter schools and three very small districts are not included.

Seven of the top eight districts for growth serve outer suburban and exurban communities, and some of the areas covered by the Lake Washington district might be considered exurban. Otherwise, it is difficult to seem much of a pattern. Some suburban districts, like Lake Washington, Bellevue, Issaquah, Puyallup and Steilacoom, are growing smartly, while others, such as University Place, Mercer Island, Monroe and Marysville, are growing slowly or shrinking.
Is this a reflection of differing growth rates within the general population? Figure 2 shows the relationship between overall population growth and enrollment in each district. (The population figures are taken from the American Community Survey five-year average estimates, so do not necessarily reflect the exact population in the enrollment year).

The correlation of population growth and enrollment growth is about 0.4, which is not especially high, given that children are distributed pretty evenly across the population. The red dot shows the average of six percent enrollment growth and nine percent population growth (hence the imbalance noted above).
The Lake Washington and Snohomish districts have just about average population growth within their boundaries, but Lake Washington saw a student increase of 19 percent while Snohomish saw a decrease of 3 percent. The Seattle district (coterminous with the city) had population growth of 14 percent but enrollment growth of just 5 percent. The Sumner district had the strongest and most balanced growth.
Variations in school district populations are driven, in large part, by cycles in real estate development. New single family home developments tend to attract young families, who place their children in local public schools. Then the children grow up, graduate and move away, leaving their neighborhoods a sea of empty nests. Then the real estate turns over and the cycle starts again.
The Sumner and White River districts are growing rapidly as the areas south of Lake Tapps fill in with new single family neighborhoods. At the other end of the cycle, shrinking districts like Marysville, Monroe and Snohomish all saw booming growth in the 1990s; the children who grew up in those areas are now gone, and their parents remain. The Bellevue district boomed in the 1950s and 1960s, crashed in the 1980s, and has been filling back up since the 1990s.
Looking Ahead
Two known, but as yet unmeasurable, factors will have a big impact on school enrollments in the next five years. First, the Millennial generation is now in prime child-rearing age, and the leading edge of that boomlet is now in the lower grades. Second, the extent to which families are moving out of central cities in response to new opportunities for remote work will determine some measure of enrollment growth in outlying districts and slower enrollment growth in central districts.
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