ATLANTA — Spurred by the threat of being held back a grade, elementary school students boosted their scores on a crucial test this year.

But middle school students continued to struggle with the Criterion-Referenced Competency Test, the curriculum exam the state uses in order to abide by the federal No Child Left Behind law.

The results are a signal that the policy of holding back third-grade and fifth-grade students who failed the exam’s reading portion is helping, state schools Superintendent Kathy Cox told board members Wednesday.

“We are now seeing unprecedented achievement in grades three, four and five,” Cox said. “It is fantastic.”

Failing students must attend a three-week summer session before retaking the test. Students who fail on a second attempt will be held back unless a parent, teacher and principal all agree the child should be promoted. Next year, eighth graders also must pass the reading portion of the test to be promoted.

The most dramatic rise among elementary students was an 8-percentage-point jump in the number of fourth-grade students who met or exceeded the CRCT’s reading standards.

The number of third-graders who failed the reading portion dropped 2 percent from last year, when about 11,000 of the state’s roughly 115,000 third-graders were unable to achieve a passing grade.

The scores for middle schools were more worrisome, she said. In particular, eighth-graders lagged behind. The percentage of students passing the reading, math, science and social studies portions of the exam all dropped