A council of Maryland educators and policy makers Tuesday approved a new model for evaluating teachers and principals that will be tried out in Prince George’s County and six other school systems this fall.

The system will tie 50 percent of future evaluations to student test scores or other student growth measures, a step that many teachers oppose. Teachers will not be rated as “effective” unless their students show progress.

Overhauling evaluations to make teachers more accountable for their students’ learning is a key part of the state’s $250 million federal Race to the Top initiative. Historically, teachers have been rated mostly on their preparation and classroom delivery.

“This is an initial step,” said Maryland schools superintendent Nancy S. Grasmick, who co-chaired the governor-appointed council. She said the state plans to refine the model based on the pilot programs’ experience next year.

The system will roll out state-wide in 2012-13, but Maryland got federal approval last week to delay high-stakes decisions about job status or tenure until the following year, once school systems have gotten familiar with the changes.

Linking teachers’ performance to tests, which were mostly designed to show proficiency at a particular time, is a new and imperfect science. Across the nation, educators and policy makers are scrambling to identify new tests or collections of class work that show how students improve within a specific teacher’s class.

The council struggled to agree on how to define fair or accurate measures. A recommendation was scheduled for the end of 2010, but the federal government approved a six-month extension.

The final proposal, which will also be used in St. Mary’s County this fall, includes traditional evaluation tools, such as classroom observations, and a list of state-approved options to chart student growth, including state tests when available. Districts will be able to develop some of their own tools.

The final vote was 13 to 7. Betty Weller, vice president for the Maryland State Education Association, said she opposed the recommendation because the final vote was rushed and many concerns and questions were not addressed.

She said she hopes the model is improved as local school systems adjust it according to their needs.

“This is not a one-size-fits-all system, even though it seems like it is,” she said.