Enrollment growth in Prince William County Public Schools slowed this year, as the district started the school year with fewer than expected kindergartners.

The official count for the state’s second largest school system was 85,055, a 1.8 percent increase over last year’s enrollment.

That’s lower than the 2.9 percent average annual growth the district has experienced over the past five years, and much lower than the nearly 5 percent annual increases it saw a decade ago before the housing bubble burst.

The actual enrollment figure was about 1 percent lower than the nearly 86,000 students projected. The discrepancy was due partly to a smaller-than-expected kindergarten class which school officials attributed to lower fertility rates tied to the recession.

They expect to see smaller incoming classes for the next two to four years. But Matthew R. Cartlidge, supervisor of planning, told the school board last week that the area’s relatively strong job market would continue to draw more families to the area.

Enrollment projections are important measures for staffing, building budgets and planning for facilities. School planners develop models that include housing construction figures and other variables.

In Loudoun County, the Sept. 30 head count was 70,855, just three students shy of the 70, 858 projection.

“Any way you round it, that’s 100 percent accuracy,” said Edgar B. Hatrick III superintendent of Loudoun County Schools.