Despite a record of mixed student achievement and financial troubles, a charter school run by the United Federation of Teachers won a two-year approval in which it must improve or close for good in 2015.

The unusual decision was made by three trustees of the State University of New York who oversee all charter schools authorized by SUNY’s Charter School Institute. Chairman Joseph Belluck noted that SUNY has a reputation as an authorizer with high standards.

“Obviously this is a serious issue and one where there’s been some significant media,” he told his fellow trustees Tuesday morning. “On other hand, this is a very complicated situation with a lot of mixed data.”

Belluck, an attorney appointed by former Gov. David Paterson, noted that the K-12 school had received the equivalent of three separate recommendations when reviewers visited in October. Their harsh critique found the elementary grades had shown enough progress to warrant a renewal. And the high school only now has its first class of seniors, meaning its data is incomplete.

But the middle school was doing so poorly that if it stood on its own, the reviewers would have recommended closure. Only 10 percent of the school’s eighth graders met the state standards in English last year.

Closing the school, or some of its grades, would have been a huge embarrassment for the union. The U.F.T. Charter School, located in the East New York section of Brooklyn, was opened in 2005 to show that charters schools and unions can work together. In 2010, the school received a short-term renewal of three years, instead of the full five, because of its mixed track record.

The decision to keep the school going for two more years was granted on the condition that the elementary, middle and high school grades meet at least half of their academic goals by 2015. The trustees also noted that the city plans to move the middle grades into the same building as the elementary grades next fall, a change that the U.F.T. says will enable them to work together.

The U.F.T. breathed a sigh of relief at the news.

“We are happy to see that the SUNY authorizers have recognized the many successes of our charter school, and have given the school the chance to build on those successes during the next two years,” union president Michael Mulgrew said in an emailed statement.

But critics of the union, which is a powerful force in Albany, questioned whether the decision was driven by politics. “We must never accept failure in our schools,” said Chandra Hayslett, communications director for StudentsFirstNY, the local chapter of the group founded by former Washington D.C. Chancellor Michelle Rhee.

“When that failure is as consistent and persistent as has been the case since the union opened its charter school in 2005, then we must be willing to stand up to powerful interests and say enough is enough,” Hayslett said.

James Merriman, chief executive officer of the New York City Charter School Center agreed with part of that criticism. He said “it would have been more consistent with standards set by the SUNY trustees to curtail the middle school grades” at the UFT Charter School. But he also said there are elementary schools with track records similar to the UFT Charter School that were renewed.

“It is high time that we have a statewide conversation about what the standard should be,” he added.

The three trustees of the SUNY Charter Schools Institute acknowledged this was a highly unusual case. Their normal procedure would be to close a charter school that’s been around for so long if it doesn’t meet the requirements for a full five-year renewal. But one trustee said the “draconian” step of closing only the middle school would penalize the remaining 700 elementary and high school students.

“Let’s hope we have a renewal application in two years as opposed to a closed school,” said Belluck.