News last week that Tony Bennett, the former Indiana superintendent of public instruction, quietly altered the state grade for a charter school founded by a campaign donor has raised questions about the validity of the trendy A-to-F grading system used to assess schools in more than a dozen states.

Bennett resigned from his job as Florida education commissioner on Thursday amid revelations that he directed staff members to alter the grade of the charter school last fall, when he was Indiana’s schools chief.

E-mails obtained and published last week by the Associated Press show that Bennett was alarmed in September 2012 when Christel House Academy, a charter school founded by Republican donor Christel DeHaan, was about to be labeled a “C” school by the state’s annual grading system. DeHaan has given more than $2.8 million to Republicans since 1998, including $130,000 to Bennett, who was elected Indiana superintendent of public instruction in 2008.

Christel House had been kindergarten through eighth grades until grades 9 and 10 were added in 2012. The new students struggled — just one-third of 10th-graders passed Algebra 1, enough to pull down the school’s overall rating from A to C. At Bennett’s direction, his staff took advantage of a regulatory loophole to toss out the performance of ninth- and 10th-graders, bringing the school’s grade to an A, according to the e-mails. Bennett later said grades for 13 other schools were similarly adjusted.

He has maintained that he intervened in the rating process because he knew Christel House to be an A school and was trying to ensure that its grade was fair.

Critics, including the teachers union in Indiana, say Bennett gave preferential treatment to a favored school. Indiana Superintendent of Public Instruction Glenda Ritz, a Democrat and former teacher who unseated Bennett in November, said her department will review the validity of the state’s grading system.

The high-profile episode underlines some of the pitfalls of grading schools.

“It should give us pause,” said Anne Hyslop, an education analyst at the New America Foundation, a left-leaning think tank. “Any accountability system should be examined, analyzed and updated as needed. You may see some states changing their A-F systems or looking for other models, but it’s a little bit too early to tell.”

Maine, which unveiled school report cards for the first time in the spring, changed the grades for three of 600 schools after errors were caught in the calculations and made those changes public, said Samantha Warren, a spokeswoman for the state education department.

“If there are legitimate things that got screwed up within the accountability system, you want to make sure everyone understands what you did and why,” said Kathy Christie of the Education Commission of the States. “You do not want to do that in a backroom. “

Florida was first to grade its schools on an A-to-F scale in 1999; then-Gov. Jeb Bush (R) touted it as a simple way to boil down complicated information for parents.

But simple isn’t always better, said RiShawn Biddle, who writes the DropoutNation blog. “It’s seductively simple,” Biddle said. “But it doesn’t provide families the information they need to be able to make decisions. If you’re a parent, you want to know growth over time. Are they providing AP courses? How are they doing in algebra? If you’ve got young black sons, you want to know: Can this school serve your son well? You can’t get that from a letter grade.”

In addition to Florida and Indiana, other states that are grading schools on an A-to-F scale include Arizona, Alabama, Louisiana, Maine, Mississippi, Oklahoma, South Carolina and Utah, along with New York City. Virginia plans to implement an A-to-F system in 2015, and Ohio intends to do so that year as well.

Many of those states made the grading system central to the accountability plan they submitted to the Obama administration to receive a waiver from the requirements of No Child Left Behind, the federal education law. Several states, including Indiana, use the grades to make decisions about funding, closure and state takeovers.

Every state with a grading system uses a different methodology, assigning varying weights to test scores, graduation rates and students’ academic growth in a given school year.

“It’s important to acknowledge these school grades represent a bundle of different judgments based on different values,” said Michael Petrilli, a senior vice president at the right-leaning Fordham Institute. “This is not science. When you get results back and they don’t match up with reality, you’ve got a problem. I think there’s going to be a good conversation about whether boiling it all down to a single grade makes sense.”