A recent report by Stanford University Center for Research on Education Outcomes found a wide variance in quality among charter schools and reported that many students in charter schools are not performing as well as students in traditional public schools.
Last week, Duncan called the report "a wake-up call, even if you dispute some of its conclusions." But Duncan's reaction shocked Bracey.
"There are numerous studies out there showing that charters, compared to public schools, don't do as well," Bracey said.
Citing the Stanford report, Duncan warned charter school management that low-performing charter schools are endangering the success of their own movement and must be turned around.
"The charter movement is putting itself at risk by allowing too many second-rate and even third-rate schools to continue to exist. Your goal should always be quality, not quantity," Duncan said.
When he was head of the Chicago public school system, Duncan helped open 70 charter schools, but he also closed down three for academic failure and mismanagement.
Duncan went on to challenge charter operators to improve accountability.
"There should be a high bar for charter approval and, in exchange for real and meaningful autonomy, there must be absolute accountability," he said, highlighting states such as Arizona, Florida, Minnesota, New Mexico, Ohio and Texas, where he said accountability is "minimal" and "unacceptable."
In 2008, 1.3 million students were enrolled in the 4,303 public charter schools operating in 40 states and Washington, D.C. -- roughly 3 percent of the amount in the nation's public schools.
The head of the nation's largest teachers union was pleased to hear Duncan's firm warnings to charters.
"They are no longer equating charters with innovation, and that was a big point for us," National Education Association president Dennis Van Roekel told ABC News Thursday. "There are a lot of charters that aren't innovative. There are a lot of public schools, non charters, that are innovative."
As the Obama administration calls for more charter schools, unions also may push for a larger role in the movement. Charter schools typically operate free from the rules inherent in union contracts and have been criticized by teachers in the past for draining resources from traditional public schools.
Last week in New York, the United Federation of Teachers and the American Federation of Teachers formalized a labor deal with Green Dot, a charter school operator, for a New York charter school in the Bronx.
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