The city’s delay in publicizing plans for its struggling schools has made strange bedfellows of an advocacy group that supports school closures and a principal who for years resisted his own school’s potential shuttering.

Families organized by StudentsFirstNY rallied outside the long-struggling Boys and Girls High School on Monday, calling on the mayor to announce a clear improvement plan for the school. It was the latest effort by StudentsFirstNY, a group that advocates for charter schools and school choice, to pressure city officials to more quickly articulate a comprehensive plan for the city’s lowest-performing schools.

Principal Bernard Gassaway made the same point when he announced his resignation late last week, blasting the city for offering incomplete plans that were “doomed to fail.” Boys and Girls, which received an “F” letter grade from the city three years in a row, has long struggled to raise test scores and graduation rates. And though Gassaway didn’t attend Monday’s rally, when schools were closed, he indicated that he didn’t mind staying involved.

“I have no problem being used for any just cause,” Gassaway said in an email. “The fact is: There is no comprehensive, strategic plan. I support all efforts to right the wrongs of this and any administration.”

Over the last few months, critics of the de Blasio administration’s education policies have shifted their focus to struggling district schools. Improvement plans for the city’s lowest-performing schools were due July 31, but the city asked for and received an extension to file those plans until next month. Thousands of families and advocates organized by the pro-charter Families for Excellent Schools pressed the mayor for a struggling-schools plan at a rally in lower Manhattan earlier this month.

But Gassaway is an unconventional choice of a champion for StudentsFirst and Families for Excellent Schools — an advocacy group which referred to the principal as a “widely respected educator” in a press release on Monday. Gassaway criticized the Bloomberg administration even as Boys and Girls avoided closure in recent years as the school’s reputation and enrollment declined during his five-year tenure. StudentsFirst and Families for Excellent Schools support closing low-performing schools.

City and state officials said last month they would not send students to Boys and Girls mid-year, and that a more detailed plan is forthcoming. Education department spokeswoman Devora Kaye said this week that Gassaway’s departure could benefit the school.

“Boys and Girls High School was moving in the wrong direction, and it was always the DOE’s intention to find a stronger leader for this school,” Kaye said Monday. Officials said the department is moving to install an interim principal.

Meanwhile, StudentsFirstNY’s director Jenny Sedlis said if the group did not see a quick response from the city, it would begin planning future rallies.

“Here we are with no principal, no leadership from City Hall, and no plan on what we’re going to do next,” said Darlene Boston, a mother of two sons who both dropped out of Boys and Girls. They were part of a class-action suit filed against the city in 2005 that charged the school with warehousing disruptive students in the school’s auditorium until they dropped out.

Howard Pressley, a senior at Boys and Girls who attended the protest with his father, said he is on track to graduate and plays on the basketball team. “A lot of people get the wrong idea about the school,” he said. “I feel like it’s a family.”

The school does have significant issues, he acknowledged. “A lot of kids barely come to school, maybe two days a week,” he said.

Patrick Wall contributed reporting.