JACKSON, Miss. (AP) — About one in five Mississippi school districts showed improvement under the state's rating system in 2016-2017, with some benefiting from better graduation rates and test scores and others apparently benefiting from changes in the scoring system.

The number of F-rated districts fell from 20 to nine, while the number of districts earning a C, B or A rating all increased, in data published Thursday.

Graduation rates rose, test scores improved and more high school students enrolled in accelerated courses statewide, which drove some districts' better test scores.

"These results reflect the progress and achievements students have made on state assessments, the ACT, advanced courses and the state's rising graduation rate," State Superintendent Carey Wright said in a statement. "Teachers, principals and district leaders have been diligent in their work to help students meet higher academic standards and achieve better outcomes."

But changes in the scoring system may have played a part in other districts' improved ratings.

The state Board of Education reset scoring levels in August after officials concluded the 2015-2016 scores included artificially high growth rates from the previous years. They re-ranked districts from top to bottom and set new thresholds. Officials compared ratings under the new and old tables, assigning the schools the higher of the two grades.

Superintendents had lobbied for using the old scoring tables, saying it was unfair to move the goalposts so late in the year, and that effort paid dividends for districts and high schools. Of the 143 districts and three charter schools rated, 28 would have gotten lower letter grades under the new baseline. Some of those districts will have to show substantial improvements in this year's results just to keep their same grades.

The boost was even bigger for high schools after state officials agreed to recalculate how they assess whether student performance improved. Although last year was the second year students took the Mississippi Academic Assessment Program tests, some high school grades still count tests students took in earlier years. Making apples-and-oranges comparisons to determine score improvements between different tests is difficult.

Elementary and middle schools, by contrast, did not benefit from the adjustments and scored lower, overall. While 20 percent of high schools got an A, only 10 percent of elementary and middle schools did. State testing chief Walt Drane said the new baseline for high schools may have to be reset again, depending on future data, because the growth numbers they'll have to hit this year to retain their grades or improve will be very high.

Still, officials said they believe the scores reflected real improvements. That certainly seemed true in districts such as Jefferson County, which bounced from an F to a C, or Amory, which jumped from a C to a high B. Amory Superintendent Ken Byars said he feels the improved grade better reflects teaching and learning in the 1,800-student northeast Mississippi district.

"We've just got to turn around and do it again," Byars said. "It's a good thing."

The news is worse for some districts, including 17 that declined by a letter grade. Another six have now been rated F for two years in a row, making them eligible to be absorbed into a special takeover district — the achievement school district — that the state is setting up. Of those six districts, Holmes County, Humphreys County, Jackson and Noxubee County also have a majority of schools rated F, one of the criteria the state has set for choosing takeover candidates.

The state's three charter schools also rated poorly, with two getting Ds and Midtown Public Charter in Jackson getting an F for a second year. Marian Schutte, executive director of the Mississippi Charter School Authorizer Board, says that although charter schools are only several years old "the charter school accountability scores and letter grades do not currently meet the authorizer board's expectations."