Updated

WASHINGTON -- Signaling some unity in the Senate on fixing the "No Child Left Behind" law, two senators announced Monday an agreement to move forward on bipartisan legislation to overhaul it.

Soon after, however, Education Secretary Arne Duncan issued a statement noting that the bill did not include a provision the administration favors, which is a requirement that local and state districts develop teacher and principal evaluation systems.

Duncan said he believes "that comprehensive evaluation system based on multiple measures, including student achievement, is essential for education reform to move forward" and "we can't retreat from reform."

Sen. Tom Harkin, D-Iowa, and Mike Enzi, Wyo., the top senators from their parties on the Senate committee with jurisdiction over education, made the announcement two days before the committee will consider the bill. Last week, Harkin released an outline of the plan the senators had been working on behind closed doors that did include a component on teacher and principal evaluations. At the time, Enzi hadn't yet publicly signed onto the plan.

Last month, President Barack Obama said he was frustrated with Congress' inability to update the law passed in 2002, so he was moving forward to allow states to apply for waivers around certain requirements. At least 39 states, in addition to the District of Columbia and Puerto Rico, have said they intend to apply for a waiver. The administration has said the waivers are a stopgap plan until Congress acts.