COLUMBIA — A much-criticized plan to start giving state public school teachers and principals letter grades received federal-government endorsement Tuesday, leading state education officials to call on the plan’s critics to embrace it.

Starting in 2014, the S.C. Department of Education hopes to launch a statewide educator evaluation system that will give teachers letter grades based in part on how well students’ performance on tests improves from year to year.

The U.S. Department of Education approved the plan, which the state developed as part of a requirement for receiving a waiver from the federal No Child Left Behind accountability law.

South Carolina is among 35 states and the District of Columbia that have sought waivers from the law that critics say sets unrealistic expectations and unfairly rates schools as failing.

The S.C. Department of Education sees the endorsement from President Barack Obama’s Cabinet agency as affirmation that the plan “wasn’t something (outside) of the mainstream” or a “radical proposal,” said state Department of Education spokesman Jay Ragley. “This is something ... that is valid.”

But statewide educator groups remain wary.

Since learning of the plan last year, educator groups and the S.C. Board of Education — which must endorse the plan before the state Education Department can launch it statewide — have raised concerns about giving teachers A through F letter grades.

About one-third of their grade will come from a measure of how much students improve on test scores compared to a statistical prediction of how well they should have performed.

Another two-thirds of a teacher’s evaluation would come from observation by principals and peers, similar to how teachers are evaluated currently.

Education groups argue that less emphasis should be placed on test scores.