No one seems to disagree that a good teacher can make a big difference in students’ education and ultimate success. A good teacher.

So it makes sense that an important task of any school district is to determine who the good teachers are — and who they aren’t.

The federal government has given South Carolina the welcome go-ahead to initiate a system of teacher evaluation based in part on how students improve on standardized tests.

That idea is unsettling for teachers who deal with students on both ends of an achievement gap and in the middle. But if students’ success is measured by tests, it is reasonable that teachers’ success is measured by their students’ performance on those tests.

In part. Not altogether.

The South Carolina plan, which will be tested before statewide implementation likely in 2014, uses student growth on tests for a third of the teacher’s assessment. The rest relies on observation by principals and peers. Teachers will be awarded a grade of A through F.

StudentsFirst fellow and local master teacher Amanda Hobson writes on today’s Commentary page that “teachers ought to be treated like professionals, which means they should be rewarded when they perform for their students, but also be accountable when they don’t.”

Other professionals are evaluated based on the results of their work: how many court victories for a lawyer; how many touchdown passes (and Super Bowl triumphs) for a quarterback; how much bottom-line growth for a businessperson.

Teachers see students anxious about getting graded — especially those students who have reason to think their grade will be low.

But testing remains a necessary means of evaluating student performance.

And until those tests reflect a vast improvement in student performance — and an accurate assessment of teachers’ work — nervous educators will simply have to persevere.

The slow pace of school improvement is unacceptable, and rewarding the best teachers while helping — or weeding out — the worst is one step in speeding up that process.

S.C. Superintendent of Education Mick Zais has invited school district superintendents to volunteer to participate in the pilot project for the new evaluation system.

He has said that he thinks good teachers are not paid enough, and poor teachers are paid too much — if, indeed, they should even keep their jobs.

Dedicated teachers are as concerned as anyone else about the sad state of too many of South Carolina’s schools.

They should see the new assessment system as a way to ensure that students are given the best instruction, and that those instructors are recognized for their good work.