Chicago Public Schools plans to build an annex for an overcrowded elite West Side school less than a mile away from an underpopulated school serving almost exclusively low-income children that once was targeted for closing.

Under a mayoral plan unveiled Wednesday at the behest of the local alderman, Mark Skinner Elementary School, known as Skinner West, will get more space while a nearby school, William H. Brown Elementary, is in line for a STEM program with new labs and teachers to focus on science, technology, engineering and math, district spokeswoman Emily Bittner said.

With classes being held in hallways and science labs, some with as many as 40 students, Skinner West desperately needs more space. The hybrid school — which admits some selective enrollment students along with kids from the booming neighborhood — has been forced to cut programs to alleviate overcrowding.

Local Ald. Walter Burnett (27th) drove a hard bargain. He was not about to agree to the Skinner expansion without getting something for Brown, a neighborhood school near the CHA’s old Henry Horner Homes that nearly became one of the record 50 public schools closed by Mayor Rahm Emanuel. On Wednesday, Emanuel unveiled a compromise with something for both schools.

“I wouldn’t allow anything to happen for one school without taking care of the other. So, we killed two birds with one stone. We took care of the one school that needs more space and the other school that needs to enhance its curriculum,” Burnett said Wednesday before the mayor’s meeting with local school councils at Brown and Skinner.

Neither proposal was mentioned in a draft of the district’s Educational Facilities Master Plan that the cash-poor CPS released late on Friday.

“CPS continues to explore possibilities for addressing overcrowded schools and improving offerings at neighborhood schools throughout the city,” Bittner said in an email, declining to say whether one of those possibilities included moving school boundaries.

Skinner’s campus of two buildings is supposed to hold about 750 students, according to CPS facilities reports. Last year, it held about 1,050, only about 26 percent of whom were considered low income, well below the district’s 86 percent average.

The top-rated school not only serves a fast-growing West Loop neighborhood, but it also can control the number of students it admits to its academically challenging classical program. Bittner said the classical program now admits 30 kindergartners per year, and last year received about 4,800 applications for 46 available spots total in grades K through eight. Skinner used to hold a citywide lottery for its magnet cluster program but had to stop accepting any of the 2,250 applications for kindergarten because it already was full of neighborhood children, she said.

Meanwhile, Brown, at 54 N. Hermitage Ave., holds about 250 students in a building that could ideally fit 630, according to CPS figures.

Brown Principal Kenya Sadler touted the STEM announcement in a schoolwide email: “Mayor Emanuel will be here announcing great changes that will occur in the school. You are welcome to join us for this amazing news. It will impact all aspects of your child’s academic experience at Brown.”

A meeting with the mayor advertised in that mass email was later announced to just be for LSC members. Reporters who arrived at Brown were barred from the meeting. A similar meeting little more than an hour later at Skinner drew more than 100 parents, teachers and children — as well as a quorum of LSC members. CPS security again would not allow reporters into the meeting.

Brown LSC member Tashonda Ford said gentrification of the neighborhood near the school was a concern for LSC members when CPS floated an idea in the past that Skinner might move some of its programs into Brown. While parents at Brown had hoped for investment from CPS for years, Ford was cautiously optimistic about both about the potential positive impact on enrollment of having a more rigorous curriculum, and whether its success might eventually limit space for the neighborhood students.

“That’s a question that goes to the demographics of the neighborhood,” Ford said. “The demographics of the neighborhood is changing. If [Brown] is more of a competitive school, [and] it is a STEM school. I do believe it’s going to draw” more students.

And parents at Skinner likewise were skeptical of the expansion plans, with several noting that the number of new homes and apartments springing up in the booming neighborhood around the school likely means that the school will outgrow the annex as soon as construction is finished.

Valerie Gomez said her second-grader was in a classroom with 36 students last year, and she doesn’t expect class size to drop much even with the annex.

“I think it’s a good move in the right direction, but with all the housing projects going up in this area, I think it will still be overcrowded,” Gomez said. “I wish the project of expansion was going to be even larger.”

Burnett denied that the STEM program is a consolation prize for Brown that makes Skinner’s expansion politically palatable.

“I don’t know what else I can do for Brown outside of making it a selective enrollment school. And if we did that, it would be for the whole city and not for the neighborhood. We didn’t want to do that. We want to keep it a school for the neighborhood,” the alderman said.

“When Henry Horner was torn down, they lost a lot of population. Now, a lot of new population has moved to the community but, because of the curriculum at Brown, parents haven’t made it a choice for their children. Now, we’re enhancing the curriculum at Brown so it becomes a better choice. It’s never enough. But for them to do work inside the building and bring in new instructors, it’s a good deal. Brown was on the list. We saved it from being closed. Now, we’re saving it some more,” he said.

Burnett said he did not know the price tag for the Skinner expansion or the STEM program, but he said the building enhancement at Brown and training for teachers would amount to a $10 million investment in the school.

CPS declined to provide the costs to taxpayers. Both projects are expected to be bankrolled with revenue from the $45 million property tax increase for school construction approved by the City Council last fall.

City Hall was tight-lipped about both projects ahead of the Wednesday night announcement — as it was recently when announcing a similar expansion at the overcrowded South Loop Elementary School, another near-downtown school with demographics resembling Skinner’s.

In an emailed statement, the mayor’s office would say only that Emanuel made it clear in his 2016 budget address that he would “invest in our schools” and he’s “following through on that commitment, starting with these meetings.”

Emanuel attended the meetings at Brown and Skinner on Wednesday, but left without talking to reporters.