Even as the Chicago Board of Education voted to expand the enrollment boundaries of a controversial new high school in the South Loop, the school’s supporters on Wednesday asked for an annex to prevent overcrowding.

The six board members unanimously approved the expansion to include Pershing Elementary School as the ninth elementary school feeding into the new high school that next fall will open in a building at 55 E. Cermak. That’s currently occupied by the top-rated National Teachers Academy, which will be phased out. Students from Holden and Armour elementary schools also now are guaranteed a spot instead of just a preference at the school whose boundary spans from the Chicago River to 35th Street.

CPS, which has no public master plan of where more schools are needed amid rapidly shrinking enrollment citywide, previously approved the school, partly to give a dedicated neighborhood high school to the Chinatown community which had lobbied for one for decades.

But after the high school for about 1,200 students was initially approved, families from the GAP neighborhood, an affluent pocket of Bronzeville, also asked to be added to the areas that will have guaranteed access to the new school. Currently they’re assigned to Phillips High School, at 244 E. Pershing Rd., that’s home to mostly low-income black students.

On Wednesday, residents of the GAP and Chinatown told board members they were worried the building will be too crowded and urged CPS to lock in funding for an annex.

Debbie Liu, an organizer for the Coalition for a Better Chinese American Community, said the nine elementary schools graduate about 700 eighth graders a year.

“There is a need to have an explicit commitment of capital funds by the city and CPS,” Liu said, beyond promises she said were made at a September community meeting by area aldermen Pat Dowell (3rd), Patrick Daley Thompson (11th), Danny Solis (25th) and Sophia King (4th) to find money for an annex. No figures were discussed on how much an annex might cost.

In urging the board approve the expansion of attendance boundaries, Dowell heralded the new school as “long needed not only by my community, but also asked for by surrounding neighborhoods.”

Leonard McGee, president of the GAP Community Organization who pushed for Pershing’s inclusion, warned that “the next bump and lump that we all need to go through … is putting an addition onto the school because it’s going to be a success because of all our involvement.”

Fellow GAP neighbor Kevin Stanciel also predicted that the new school “will be overcrowded because of the demand of it, and from my years as a CPS parent I understand how long things take. … I would ask that you start right now on planning for an expansion of that school.”

With the same vote, the school board also decreased attendance boundaries for the shrinking Phillips, and Tilden High School, whose principal was just promoted to Chief Equity Officer in the central office.

The parent group Raise Your Hand criticized the lack of planning in a tweet, saying that “the building that now includes a thriving NTA ES will be closed to accommodate way too big boundaries.”

CPS spokesman Michael Passman said CPS, in making it decision to expand the attendance boundaries, studied enrollment patterns at other in-demand neighborhood schools like Lincoln Park High School, which enrolls about 46 percent of kids living in its boundary.

“We expect the Near South High School to be efficiently utilized when it is fully enrolled,” he said.