Out of 300,000 teachers in California, "No one … can tell you who's in the top 10 percent and who's in the bottom 10 percent. Something's wrong with that picture," Duncan said on ABC's "This Week With Christiane Amanpour" program.
Duncan also advocated for adding merit pay for teachers to teach in difficult districts.
"We've had very few incentives and, frankly, lots of disincentives for the hardest working, the most committed teachers and principals to go to inner-city communities, to go to rural communities, to go to the children in the neighborhoods who need the most help," he said.
"And let me be clear: Financial incentives are a piece of that, but a small piece," he said.