Last year, Rep. Glen Casada proposed that all Tennessee schools be assigned a single A-F grade starting in the 2017-18 school year. His bill became law — but now he wants it delayed.

The Williamson County Republican and House majority leader says the state should hold off because this is the first year that elementary and middle school students will take its new test.

Last spring, the state canceled TNReady for grades 3-8, prolonging the transition to an assessment that is supposed to be more rigorous. The cancellation came just a month after Gov. Bill Haslam signed the law requiring the Tennessee Department of Education to develop a school grading system by this fall. Sen. Dolores Gresham, a Republican from Somerville, sponsored the bill in the Senate.

“I’m a big believer in transparency,” Casada said this week, “but this year’s data is not up to standard.”

Casada said that Department of Education officials are moving ahead with the grading system, which is now built into the state’s plan to comply with the new federal Every Student Succeeds Act, or ESSA, but are listening to feedback.

Officials from Williamson County Schools, the system in Casada’s district, are among school leaders with a separate set of concerns. They worry that under the new A-F grading system schools with high test scores will post lower grades if they don’t show growth. The school board in Collierville, a wealthy Memphis suburb, voted this week to lobby against the new grading system for the same reasons.

That’s the inverse of concerns discussed when the bill was snaking through the legislature last year. Then, supporters considered it a common-sense aid to help parents understand school quality. Because test scores often correlate with wealth, critics charged that schools in affluent, high-performing districts like Williamson would always get As, and that schools with more poor students would get lower grades.

The Tennessee Department of Education officials took such concerns into account when they drafted the proposal for the grading system, which was part of their ESSA draft plan. ESSA focuses more on school-level accountability than its predecessor, No Child Left Behind. In the draft, both growth and raw achievement would be part of schools’ final grade, with the idea that schools won’t have an edge because of their demographics.

The performance of subgroups such as racial minorities also will be a determining factor in the ratings.

School leaders nationwide are facing the challenge of conveying school quality with a single letter grade as states and cities move to adopt such grading systems. Earlier this week, leaders of high-achieving districts in Texas urged state officials to repeal a new A-F system, but their lieutenant governor said it’s here to stay.

Michael Petrilli, the president of the Fordham Institute, says developing a fair and relevant grading system is a balancing act.

“If high-achieving schools end up getting low grades, people won’t trust the grading system,” he said.

But Tennessee officials say that historically high-ranked schools have nothing to worry about under the state’s proposal and that all schools will have the opportunity to make an A.

“… We were intentional about engaging all types of districts across Tennessee because this framework has to be able to work for the variety of schools we have,” said department spokeswoman Sara Gast.