U.S. Secretary of Education Arne Duncan will call today for a new process allowing states that have gotten waivers from parts of the Elementary and Secondary Education Act some flexibility in using student achievement to evaluate teachers, sources say.

Teacher evaluation based in part on student outcomes (i.e. test scores) has been the most difficult piece of the waiver framework. And it’s become even more complicated as states begin to embrace the Common Core State Standards, which mean new, higher expectations for students.

The tests that are being developed by two state consortia to go along with those standards won’t be ready until the 2014-15 school year. In the meantime, many states are using their own common-core-aligned tests.

Specifically, Duncan is planning to allow waiver states to make the case that they need an additional year to factor student achievement into personnel decisions. That will give states time to put the common core in place.

The announcement comes after the American Federation of Teachers called earlier this year for a moratorium on high stakes testing until states have had time to implement the standards and new tests. And the Council of Chief State School Officers has also called for additional flexibility as states roll out the common core.

More details are expected this afternoon. Watch this space.