A group of Park Slope parents is in an uproar over the city’s plan to build a new school building that they say will house two “separate but equal” elementary schools. But schools officials say the plan is exactly how community leaders wanted it.

The plan would replace PS 133’s century-old school building in North Park Slope with a brand new building on the same site. The catch is that the shiny new space will house not just PS 133 but also a new school whose students are likely to be whiter, more affluent, and better prepared for school.

That’s because in an unusual arrangement, the two schools will belong to different districts. PS 133 is located in District 13, which stretches from Brooklyn Heights to Crown Heights. The second school, slated to be twice the size of PS 133, will be part of District 15, which begins just blocks away, and is intended to reduce crowding at PS 321, which is 62 percent white and has only a small fraction of students eligible for free lunch. On average, students in District 15 perform better on state tests than students in District 13.

Parents and community activists say the presence of two separate schools with different demographic compositions would amount to a regression to the days of racial segregation. Via e-mail and Twitter, they are bombarding schools officials and City Council members from the area with requests for a different use of the new building.

“This is an issue that demands creative leadership from you and Councilman [Bill] DeBlasio,” a District 13 mother, Paget Walker, wrote to City Councilman David Yassky. Both Yassky and DeBlasio are running for citywide positions. “In this election year, our candidates need to let us know that they will not make a politically expedient compromise. Our community depends on you.”

So far, the parents haven’t heard back, although at least one has been invited to political fundraisers, according to Rob Underwood, the father of a soon-to-be kindergartner who has been spearheading the campaign.

“If they did a single mixed school it could both address the D15 problem in terms of overcrowding and also the longer term root cause: [the need for] better school choice in the overall area,” Underwood said.

The city did initially proposed a single school with a zone spanning district lines, but two sources familiar with the situation said parents from District 13 rejected that proposal. Members of the district’s parent council opposed the proposal, according to Department of Education spokesman Will Havemann. The council has not responded to e-mails and phone calls this week.

Jim Devor, a member of District 15’s parent council, said members of District 13’s council “had kittens” when they were presented with the possibility of losing control over one of the schools in the district. He said District 15’s council went along with the compromise of having a divided school in order to get the city to agree to build other schools in other overcrowded parts of the district.

Immediately afterwards, Devor said, District 15’s CEC raised the alarm about possible segregation in the new building.

“I’m not sure that acquiescing to D13’s insistence was necessarily in the best interest of the entire community,” Devor said.

Havemann said the two-school solution will let the department meet two major goals. “This situation has absolutely nothing to do with race,” he said. “It has to do with a compromise that will provide PS 133 with a desperately needed new facility” and also reduce overcrowding in District 15.

At public hearings earlier this spring, the loudest critics of the plan were Park Slope residents who did not want to see PS 133’s Victorian building torn down and its 30-year-old community garden relocated. The people who are vocally weighing in now say they want to see a school built, as long as it has an equitable distribution of students.

The actual programs that will go into the new building will be decided sooner to its opening date in 2012, Havemann said. What was approved this spring was the architectural plan for the new building, which includes two separate entrances for the two schools.