Class size is one of the key issues that has emerged in the heated race between incumbent Jane Strauss and challenger Louise Epstein for the Fairfax County School Board’s Dranesville seat.

In a nutshell, schools with high proportions of poor children and non-native English speakers get more teachers and thus lower class sizes than their more affluent counterparts.

This is called “needs-based staffing,” and its proponents say it’s an efficient, flexible way to make sure that extra resources are finding their way to needy kids as those kids move from school to school.

Critics of the needs-based staffing formula — such as Epstein’s campaign manager, Catherine Lorenze — say it’s not fair because some kids, by virtue of their school’s affluent, native-English-speaking demographics, end up in larger classes than other kids.

Behind the back-and-forth is a fundamental question about what it means to distribute resources fairly in public schools.

But it’s hard to think about that fundamental question without knowing something about the hard numbers.

The cost of needs-based staffing across the county: $45.5 million in fiscal year 2011, or approximately two percent of the school system’s $2.2 billion budget.

A table of average class sizes at elementary schools around the county:

Truly geeky readers might want to know what the needs-based staffing formula is, exactly.

First what it is not. It is not the county’s priority schools initiative, which is a separate endeavor that doesn’t much impact class size. More on priority schools in a future post.

The staffing formula is complicated and differs among elementary, middle and high schools. But the basic idea is that every school is allotted a certain number of teachers based on the county’s teacher-student ratio (26.5 for elementary school).

Then, administrators assign additional teachers to schools based on the number and proportion of needy kids.

Perhaps the best way to understand needs-based staffing is to look at a concrete example.

Below is a slide that was presented at a 2010 Superintendent’s Parent Advisory Council meeting. It compares staffing at two schools with different demographics.

Whether the staffing formula stands as-is may depend on the outcome of the Nov. 8 school board election. In Dranesville, Strauss has supported the formula while Epstein has said she will work to revise it so that the distribution of teachers is more even around the county.