Charter schools that play by Mayor Bill de Blasio’s rules will get a leg up in the rush for coveted public school space, the mayor said on Thursday.

In his most expansive remarks on charter schools since March, the mayor said he would will soon set a “clear standard” for charter schools that want space in public school buildings. He indicated that he could try to measure schools by how they serve high-needs students, their student retention rates, and even how much they “teach to the test.”

De Blasio’s remarks are significant because they provide a window into how his administration may seek to interpret a new charter school facilities law that could be costly for the city. Although it requires the city to provide new or expanding charter schools (at least 17 are hoping to open in 2015) with city-funded access to facilities, the mayor doesn’t have to give them space inside of public school buildings, which many charter schools prefer because it provides access to a school’s cafeteria, gymnasium and auditorium, among other reasons. Instead, the mayor could force them operate in private space—with the city footing the bill.

“The thing to recognize is our school system has finite physical space, and so by definition there will be a series of choices we have to make and not everyone will get the space they want,” de Blasio said Thursday at a charter school in East Harlem, the final stop on his five-borough tour on the first day of school.

De Blasio has yet to detail exactly what “co-location guidelines” he’ll ask charter schools to follow, though he said Thursday that a full policy is coming soon. A working group overseen by Deputy Mayor Richard Buery and Chancellor Carmen Fariña has been advising de Blasio on the issue.

But de Blasio offered a some clues into his thinking at Thursday’s press conference, which took place at Amber Charter School, a unionized school which operates in private space and has close ties to the East Harlem community. Amber “is a charter school that really exemplifies our values,” de Blasio said.

De Blasio then ticked off a series of critiques about enrollment and instructional practices that are often associated with charter schools. Unmentioned were the specific charter schools usually at the center of those critiques, which are typically part of large charter management organizations like Success Academy, KIPP, Achievement First, and Uncommon Schools.

Although charter schools serve higher-than-average numbers of black and Hispanic students, some lag behind in high-need student categories. To operate in public space, de Blasio said, schools would need to be “inclusive” of special-education students and English language learners. He also indicated that they would have to do a better job of serving more students in older grades, an issue that has divided the charter school sector.

“It’s very important that charters retain the students they start with, whether they’re academically strong or academically challenged,” de Blasio said.

De Blasio also suggested that the administration might take a look charter schools’ instructional practices when deciding which schools get preference for space.

“It’s very important to us to not just dwell on standardized testing and not just focus on teaching to the test but focus on multiple measure and teaching critical thinking,” he said.

The law also created a legal process to settle space-planning disagreements between de Blasio and charter schools. Speaking about the process, de Blasio suggested he was prepared to defend his co-location rules.

“Where we have a difference … there’s a due-process dynamic to the state law, including going into the judicial system,” de Blasio said.