A resolution to move an Upper West Side middle school passed on Wednesday night, but not before Cynthia Nixon — “Sex and the City” actress, Alliance for Quality Education spokeswoman, and parent at the school — was shouted down briefly during a heated public comment session.

Nixon was stepping into a fight that has been raging on the Upper West Side for months. The fight began as a discussion about how to deal with overcrowding at public schools but has spiraled into a raging debate about class and race and privilege in Upper Manhattan. Confrontations have gotten incredibly emotional — and personal: On this site, a commenter posing as Cynthia Nixon’s fictional son, Brady, from “Sex and the City” accused his “mom” of hypocrisy. And parents at Nixon’s school, called the Center School, have charged another school’s parents with racism and class prejudice, citing postings from last January on the Urban Baby Web site that called Center School students “thugs.”

At issue is a plan that would move the Center School from its current home inside a larger elementary school on West 70th Street, PS 199. Supporters of the plan tout it as an easy way to relieve crowding at the elementary school, which is growing so quickly that parents fear it will not have room to hold their younger children. Opponents, including Nixon, argue that moving the Center School exacerbates segregation by race and class. (PS 199, a zoned school, is two-thirds white, while the Center School, which draws its students from throughout the district, is half white and has a higher proportion of black and Hispanic students.)

If the plan becomes official, which it almost certainly will after Wednesday’s vote, the Center School will move to another school building several blocks away.

Nixon and other Center School parents have vehemently opposed the plan for months, making fliers and using the school’s Web site to organize protests. They also delivered passionate testimony at the meeting Wednesday, choosing Nixon and another mother to represent their cause. In her short remarks, which I captured in the video above, Nixon argued that there is a stark difference between the demographic of the Center School and the “increasingly white and increasingly affluent” elementary school it shares space with. Moving the Center School away, she said, would lead to a “de facto segregated building on 70th Street.”

The Upper West Side school war began in September, when the city Department of Education suggested two plans for how the Upper West Side could relieve crowding.

One would have moved 30 percent of students to new schools. But the local parent council that has final authority over zoning matters last week indicated that it would back a much tamer plan. That one would move only a handful of students, keep siblings in the same school, and, most controversially, relocate two schools. One of those schools, Anderson, a gifted school that pulls students from across the city, agreed to a move. The other, the Center School, where Cynthia Nixon is a parent, has spent weeks fighting tooth and nail against the plan.

The people booing Nixon were led by a growing group of parents who are zoned for PS 199 but fear that increasing crowding could make the school too packed to have room for their children. If the Center School moves out of their building, that will shore up space for their children at PS 199. These parents, who have maintained a Web site that some say contains misinformation, turned out in large numbers to the meeting on Wednesday. (Below the jump, view a video of their spokesman, Eric Shuffler, speaking out at the meeting; he, too, was booed.)

But Nixon’s contingent was by far the largest. It included not only by Center School parents but also parents from at least four neighborhood schools, who echoed Nixon’s argument about diversity. The group walked out in protest as the council prepared to vote. A number of PS 199 parents who said they supported the Center School joined them.

Also walking out — at times to shouts of “Yes, we can” — were parents from the Computer School, a middle school whose building will be Anderson’s new home, and PS 75, a diverse elementary school whose zone was trimmed in the resolution.

Council members said they had no authority to involve issues of diversity in the rezoning process. “The [Community Education Council] does value diversity. We’ve talked about it,” CEC 3 member Jennifer Freeman told me after the meeting. “We were working with the tools available to us so the main topic in this conversation had to be overcrowding. We would welcome the opportunity to talk about diversity more.”

During the meeting, one council member explained that she wanted to deal with issues of race and class segregation in the district but that now was not the right time to do so.

“If not now, when?” audience members shouted at her.

That council member, Danielle Moss Lee, ultimately abstained from voting. She was the only council member present who did not vote in favor of the resolution.