Hundreds of people crowded into Brooklyn’s I.S. 281 last October for a public hearing on a plan to install a new charter school in the building. More than 50 people spoke that night, and all but a handful said they opposed the space-sharing proposal.

Later that month, as a school policy board appeared set to vote in favor of the plan, an I.S. 281 teacher said she was hitching her wagon to the mayoral frontrunner, Bill de Blasio, who had promised to review each approved co-location.

“Hopefully,” the teacher said, “de Blasio does what he says he’s going to do.”

But after de Blasio, now mayor, announced last week that almost all Bloomberg-era co-location plans would go forward, people at I.S. 281 and across the city who had pinned their hopes on him said they were disappointed. Parents and politicians alike said de Blasio was already breaking a promise to consider public input when making school decisions.

“We expected to see change,” said Laurie Windsor, president of the elected parent council that oversees I.S. 281’s district. “That’s what they kept saying, and we really, fully trusted them. Now the trust is gone.”

On Thursday, de Blasio and his schools chief announced that they had reviewed 45 co-locations approved in the final weeks of the Bloomberg administration and decided to cancel nine, including three that involved charter schools. The three dozen other co-locations, including the one at I.S. 281, will proceed as planned.

“We did a thorough analysis as quickly as we could in the first weeks of being here,” the mayor said at a press conference Thursday. “We decided that some of these were not fair, did not make sense, and we took action.” (Education department officials also noted that overturning many of the proposals would hurt students who had applied to attend the new schools.)

De Blasio’s announcement outraged supporters of the three Success Academy charter schools that lost space they had been promised. But it also inflamed many parents, educators, and elected officials who had counted on the new mayor to reverse more co-locations, and who saw in his decision a lack of consistency and transparency. The backlash revealed how quickly the administration’s supporters might morph into critics if their hopes for specific causes are dashed.

Vincent Gentile, a south Brooklyn City Council member who supported de Blasio’s mayoral bid and lobbied against the I.S. 281 co-location last fall, said the city had not contacted him or others invested in the school during the review process.

“To my knowledge, none of that input was sought,” he said, adding that he learned of the decision only when it was announced publicly. “Part of the disappointment was the lack of a heads up.”

Gentile and two other Democratic City Council members released a statement Thursday saying they were “furious” with the decision and intend to fight it “tooth and nail.” Separately, the parent groups behind a lawsuit meant to stop more the co-locations said they would continue with their legal challenge.

The principal of a school that will share space with a new charter school next fall said he emailed a deputy chancellor during the review period to raise his concerns about the co-location, but did not get a reply.

“I didn’t find the review to be a transparent process,” said the principal, who asked for anonymity because a superintendent instructed him not to discuss the decision publicly. “There wasn’t a way for principals to make their case.”

Last week, the education department announced that it would more actively involve the public in future co-location decisions through extra meetings and the formation of a working group. But the effort was seen by some as a half-measure, since the new engagement policies did not apply to the pending co-locations.

“It was like, ‘We’re going to listen to you in the future, but this is what we’re doing now,’” said Larry Acosta, who directs an adult education center based inside I.S. 171, a middle school in East New York that will have a new middle school open in its building this fall.

At least one group endorsed the city’s co-location decisions: a coalition of charter schools seeking the city’s support and whose leaders were invited to meet with top city officials shortly before Thursday’s announcement. They released a statement the next day calling the city’s decision-making process “thorough” and “principled.”

But such support was drowned out by criticism from people who felt betrayed by de Blasio, who during the mayoral campaign had pledged to “rescind those [co-location] proposals that have clear negative impacts,” while allowing the rest to remain. Many people appeared to have disregarded the second half of de Blasio’s promise.

“He’s a scam,” said Josephine Shayef, whose son is a seventh-grade student at I.S. 281. “He told us when he was elected, everything would be reversed. Why did he lie?”

For Mona Davids, the head of the New York City Parents Union, the administration’s decision felt like a personal betrayal. Her organization is a part of a lawsuit alleging the Bloomberg administration acted unlawfully in approving dozens of co-locations last October, and Davids was expecting big change from de Blasio.

Instead, the limited scope of the changes left her “hugely disappointed and surprised.”

“As much as de Blasio says he’s about a new day, he’s not engaging parents,” Davids said. “At this point, de Blasio continues to renege on all campaign promises.”

Windsor, the CEC president, said parents and community members will now regard the administration’s school policies with more skepticism.

“Now you have to take it with a huge grain of salt,” she said.

But Dionne Grayman, the co-founder of NYCpublic.org, an organizing tool for public school parents, defended the city’s decisions, saying it would have been infeasible to undo all the planned co-locations. She added that it will take time for the city to better include parents in its decision-making, but even then parents will not always be satisfied with the city’s choices.

“As optimistic as I am, I think we have to be realistic,” she said.

Sarah Darville contributed reporting.