Charter schools can be viewed through multiple lenses when comparing them to traditional public schools: academics, innovation, cost. Another lens with which to assess the value of charters is how equitable they are in their choice of students. When the Education Law Center (ELC) recently scrutinized student populations , they found that charters were not serving as broad a range of students as traditional district schools. Among its findings: that charters are more economically and racially segregated than district schools, and serve a more financially advantaged population. In addition, charters serve fewer English-language learners than district schools. And, they serve a lower percentage of students with severe disabilities.

These are important findings because, according to the report, 33 percent of Philadelphia’s kids are enrolled in charter schools and 31 percent of total public school spending goes to charters. Comparisons of academic performance of charters and traditional public schools is a matter of debate. Charter supporters say they’re better; detractors say that charters “cream” students, finding ways to reject more challenging students.

Notably, one of the reasons it’s hard to compare academics precisely is that there are variances in the populations of charters and district schools. The ELC report confirms that, suggesting charters are less equitable and less diverse than they should be.

It found that more than half of charters are hyper-segregated with more than two-thirds of students from a single racial group. Imbalances were also found in enrollments of English-language learners — with district schools serving three times as many English learners as charter schools.

Most troublesome is the fact that charter schools serve high percentage of students with common disabilities that don’t require high cost aids and other services, and a low percentage that require higher cost aids and services.

That’s significant because of the way the state funds special education. Until relatively recently, the state figured a flat 16 percent of students in any given school were disabled and so budgets were set accordingly, allocating this year $29,299 per special ed student (vs. $9,099 for non-special education). In 2013, the state changed the funding formula; district schools are funded according to three tiers that better reflect some disabilities require less expensive aids than others.

Meanwhile, charters remain part of the old funding formula, receiving the per-pupil flat rate, regardless of the cost of providing services. As the report points out, this means charters might be incentivized to accept more students with less severe disabilities that cost less to accommodate.

These imbalances matter for a number of reasons — chief among them is that they disrupt the equity that public education must provide to every student.

The district, which approves charters, should add equity as one of the criteria for approving or renewing charters. Also, at the state level, the funding formula for special education needs to be updated.

Big cities such as Philadelphia have long battled the educational divide between its population and more affluent suburbs. Charter schools, intended to enhance the offerings of the district, should not be the source of a new divide.