Former U.S. Secretary of Education Arne Duncan, in an open letter to college presidents and education school deans, says the system for training teachers "lacks rigor, is out of step with the times," and leaves them "unprepared and their future students at risk."

His letter was posted Tuesday on the website of the Brookings Institution, a Washington-based think tank where he is a nonresident senior fellow.

Duncan, said a Brookings' spokeswoman, hopes to spark a conversation about teacher-preparation programs, something he also tried when he led the Education Department. The criticism is hardly new and comes as fewer students are entering teacher-preparation programs nationally.

"We should ensure that they're held to high standards like engineering, business, and medical students," Duncan wrote of prospective teachers, "and we should only be giving the best grades to those teacher candidates who are most prepared for the classroom."

Duncan, who oversaw education under President Obama from 2009 until earlier this year, cited a 2014 report by the National Council on Teacher Quality that found some of the biggest teacher-training programs are twice as likely to graduate students with honors than other programs at those colleges. About 30 percent of all students graduated with honors, compared with 44 percent of education majors, the study found.

Duncan cited several local schools, including Cedar Crest College in Allentown, where, he said, 80 percent of education students graduated with honors, compared with 26 percent of all students, and Pennsylvania State University's Harrisburg campus, which had a 44-point gap. The report also showed gaps at several of Pennsylvania's state universities, including West Chester and Bloomsburg Universities. Temple and Drexel Universities did not show gaps.

"There can only be two explanations for this unsettling phenomenon: Either your teacher-training programs are attracting an unusually gifted group of students, or the standard for honors in education is too low," Duncan wrote.

Some local educators took sharp exception to Duncan's critique, saying that he unfairly based his observations on one report and that relying on letter grades to evaluate programs is ill-advised.

"He actually has a very narrow perspective," and an outdated one, said Kenneth Witmer Jr., dean of West Chester's school of education and social work. "His article sounds like something my grandfather would have written."

Witmer said the criticism was unfair to West Chester, which has a requirement that students entering the major have a 3.0 grade point average. It makes sense, he said, that students in the program get higher grades, because those with lower grades already have been weeded out.

Gregory Anderson, dean of Temple's education school, acknowledged that not all programs are operating at their best, but disagreed with Duncan's sweeping statement.

"He's not wrong to question some of the outcomes," Anderson said. "The critique is valid. I just think it's overblown."

Duncan singled out Hunter College in New York City for using more vigorous evaluation criteria.

The college "requires their teacher candidates to record themselves teaching on video and then later document and analyze their own instruction; this allows professors to see both the instruction choices and the candidates' analysis of their work," he wrote.

Pam Grossman, dean of the University of Pennsylvania's graduate school of education, said other schools, including Penn, are doing that kind of evaluation. Penn also requires teacher candidates to spend 900 hours in classrooms working with teachers - twice the national average, she said.

Lee Ann Wentzel, superintendent of the 5,555-student Ridley School District in Delaware County, which has 420 teachers, said Duncan's comments were "a little offensive."

"The teachers that come to us come in prepared. There are a whole lot fewer of them because of comments like this," she said.

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