Nearly half of Chicago’s schools are under-enrolled, according to new data that highlights the impact of the city’s population loss over the past decade.

The district uses capacity calculations to gauge how effectively schools are using their building space and  which campuses need new additions. In years past, the numbers have been used to help justify school closings.

Chicago Public Schools found 13 more schools operating below capacity than last year. However, recent changes to district metrics make apples-to-apples comparisons difficult.

Some 233 schools don’t have enough students to fill the building to capacity, according to the district, and 34 schools are considered overcrowded, out of about 480 schools. The total number of schools does not include schools that share a building with other schools.

The district, which released its analysis over the holidays, also announced tweaks to its so-called space utilization formula — removing from its analysis pre-K classrooms, as well as rooms that are 650 square feet or less. The district also noted that it gave principals the chance to suggest changes to their school’s designation, noting in a statement that “capturing and responding to principal and community feedback is a key priority for the district.”

Critics, however, question whether the formula is an accurate assessment of school capacity.

“We think this data belongs in the trash can,” said Erica Clark, a founding member of the advocacy group Parents 4 Teachers, noting that several schools shown to use their space efficiently are, by her estimation, actually overcrowded. “It’s flawed and based on faulty assumptions.”

While changes to the formula were intended to more accurately capture how schools use their space, Wendy Katten, director of strategy for the parent-led advocacy group Raise Your Hand for Illinois Public Education, said she didn’t see any significant improvement to this year’s metrics.

“It doesn’t really give you a clear picture of what is happening in the building,” Katten said. The district’s calculation uses 30 students in a classroom as the ideal number, but Katten said that number is far too high, tantamount to “a slap in the face to evidence-based policy making.”

The Center for Public Education cites research showing that as classroom sizes fall, student achievement rises, and the ideal class has no more than 18 students.

But district officials noted that 30 students per class was the average number of students in a full classroom. “The 30 student figure is set in alignment with the district’s class size policy, which is negotiated as part of the CTU [Chicago Teachers Union] contract,” said Michael Passman, spokesperson with the district.

Waters Elementary, a school in the Ravenswood Gardens neighborhood, was labeled ‘efficiently’ enrolled, based on data from the past two school years. But that school is slated for a $24 million expansion.

Josh Kalov, a former Local School Council member at Waters and the creator of Apples2Apples, which analyzes CPS utilization data, said the school was so full that it hasn’t been able to host pre-K in recent years.

“If you assume that it’s a properly utilized elementary building, each school would have x number of pre-K classrooms,” he said, noting that CPS data “isn’t the only thing to look at.”

Some proponents of neighborhood schools, meanwhile, fear that the growing number of schools labeled underutilized means those campuses could be targeted for closures since a five-year moratorium on shuttering district-run schools ended last year.

“I’m concerned that CPS is beginning to use enrollment data and utilization data to build the argument for multiple school closures next fall,” said Alexios Rosario-Moore, a research and policy associate with Generation All, a nonprofit that advocates for neighborhood schools.

Chicago closed 49 schools in 2013, the largest school closing in American history, relying primarily on a justification that schools were underperforming and under-enrolled — there are currently 77,275 students less students today than in 2002. But several years after the closings, research has shown that the traumatic impact of multiple school closures on generations of students didn’t help their academic success.

Since then, parent advocates, community members and even state legislators have pushed for alternatives to school closings to deal with under-enrolled schools.

A state law enacted in August required Chicago to propose actions other than closure for schools where enrollment is declining. The board approved the policy in October of last year.

Earlier this year, the district also released its Annual Regional Analysis, a controversial report about enrollment, academic options, and quality at schools throughout Chicago. The report divides the city into 16 “planning regions,” and shows that in many places, students are skipping out on nearby options, with fewer than half of district students attending their designated neighborhood schools.

At a November meeting Chicago Public Schools convened in a predominantly black slice of the Far South Side, residents pushed the district to do more to engage families and to help dispel stigmas they say make their campuses a tough pitch to prospective families. At another such meeting, held recently in a mostly Latino swath of the city, parents called for more resources, such as counselors and bilingual teachers, for neighborhood schools.

Chicago Teachers Union Vice President Stacey Davis Gates said CPS should provide more funding for schools to use additional space for parent rooms or other community spaces. “Building spaces would be “utilized” as such if CPS provided funding for community schools,” said Gates. “The real problem is racist city policies that have led to under-enrollment in some areas and overcrowding in others.”

The future of Chicago schools amid declining enrollment and a rising number of families opting out of their neighborhood schools for magnet or charter schools has galvanized city and state leaders alike.

Daniel Anello, CEO of Kids First Chicago, a school-choice group which compiled the Annual Regional Analysis with the district, said the drop in Chicago’s school-age population pointed to questions beyond the capacity of individual buildings.

“What is the academic model of the future — and is the utilization calculation based off of thinking ‘what does it take to have a world-class school?’” Anello said.