As the city faces increasing pressure to promote school diversity, a new report shows that more districts nationwide are using socioeconomic status to shape admissions processes.

Ninety-one districts and charter networks now have at least one school that factors socioeconomic status into its assignments, according to a report released Tuesday by the Century Foundation. The number identified by the foundation has more than doubled since 2007 and represents about 4 million students nationwide, the report says.

The report comes after the city enacted a law to report school diversity statistics and more recent controversies over school-zone changes, and in the wake of a widely reported 2014 study showing New York City had one of the country’s most segregated school systems. In November, the city announced a pilot program designed to increase diversity at a handful of elementary schools.

Though the city has been reluctant to go further, this report suggests a menu of options that could be employed if it decided to make other policy changes. (The report does not assess whether these measures succeeded, however.)

“There is some momentum for this work and a greater policy toolkit for what districts are trying,” said Halley Potter, the co-author of the report who works as a researcher at the Century Foundation.

The admissions options vary. A magnet school in the Boulder Valley School District in Colorado made the list, as did the entire district of Cambridge, Mass., which aims to achieve school diversity with a citywide formula.

Several school districts in New York City made the cut for participating in the elementary-school pilot program, which will allow schools to reserve seats for low-income students including Brooklyn New School, Castle Bridge School in Washington Heights, and the Neighborhood School in the East Village. P.S. 133 in Brooklyn, which admits students from two neighboring districts in an effort to maintain diversity, was also included.

Simply considering a student’s socioeconomic status in admissions does not make a school diverse, Potter warns. In fact, the report suggests that three of the common strategies — magnet school programs, charter school admission systems designed to draw diverse students, and transfer policies that consider diversity — have less potential than other strategies to promote district-wide integration, though they can be effective at the school level.

“It’s a separate and more important question to ask, how big are each of those steps?” Potter said.

Potter chose to focus on socioeconomic integration, in part, because using race explicitly is legally tricky. Federal guidance allows school districts to adopt race-based integration strategies, but only after they have considered race-neutral options. Socioeconomic status can be a good proxy for racial diversity, since a high proportion of low-income students are black or Hispanic. (While all of the schools in this report considered socioeconomic diversity, some have also adopted race-based enrollment strategies.)

Of the five methods of integration detailed in the report, rezoning is the most frequently used. Most public schools nationwide, including most elementary schools in New York City, enroll all students that live within a geographic area. If families of similar income levels and backgrounds live near each other, this creates schools with student populations of similar race and class.

Redrawing the lines with careful attention to the socioeconomic makeup of surrounding neighborhoods can help balance diversity in schools. Of the 91 districts and charter networks in the report, 38 employed this strategy.

As more districts adopt diversity policies, the Obama administration is expected to ask for a $120 million grant program Tuesday in its final budget that would help schools become more integrated, according to Education Week. The program could be similar to the socioeconomic integration pilot program started in New York by now-U.S. Education Secretary John King.

Potter said that the attention at the federal level, coupled with policy shifts around the country, could create a friendlier environment for integration policy experiments.

“Hopefully those two ingredients together will be a good mix,” Potter said.