The ongoing audit of Shelby County Schools’ high school records has garnered renewed interest after the principal who first reported possible grading irregularities resigned abruptly last week from Trezevant High School, a football powerhouse in Memphis.

But investigations into grading and testing practices are fairly common across the nation, and not unheard of in Tennessee.

Last year, Metropolitan Nashville Public Schools launched an internal investigation into allegations that educators were pulling low-performing students out of testing to boost the image of several struggling schools. The review found no widespread violations, but warned that “the state’s current calculation of on-time graduation rate in a four-year period puts increased pressure on teachers and students to pass classes and earn credits.”

Districts with many impoverished students who lag by several grades are particularly susceptible to allegations of cheating in an era of high-stakes testing and accountability, said Erich Martel, a retired teacher and whistleblower for grading scams that surfaced in Washington, D.C. in 2001.

“A lot of students come into school and they are poorly socialized for the school learning environment. But because there is pressure to promote, students are promoted,” Martel said. The result is that “students who have not mastered the requirements from the previous grade … are expected to master the next higher grade.”

The independent audit of Memphis high schools launched earlier this year after Trezevant Principal Ronnie Mackin reported inaccuracies and inconsistencies in transcripts and report cards.

Those irregularities were highlighted again last week when Mackin submitted a resignation letter charging a district cover-up of an alleged grading scandal — an accusation that Superintendent Dorsey Hopson has denied.

In his seven-page, single-spaced letter, Mackin also alleged that other schools, particularly in the district’s high-profile school turnaround program known as the Innovation Zone, are altering grades as well.

The district’s independent audit, ordered by Shelby County Schools in consultation with the State Department of Education, is being conducted by a North Carolina-based CPA firm. Among other things, it’s looking into evidence that Mackin turned over to district leaders last fall of altered grades for several athletes on Trezevant’s championship football team, along with most of the senior class.

“While there was no evidence that any other schools had discrepancies in student transcripts, the District and Tennessee Department of Education agreed a proactive audit was necessary to ensure all student records were being handled properly,” Shelby County Schools said in a statement Monday.

Below is a timeline of events in the case:

April 2016 — Ronnie Mackin named principal of Trezevant High School for the 2016-17 school year

September 2016 — Mackin reports possible grading irregularities to district leaders

September 30, 2016 — Superintendent Dorsey Hopson alerts the Tennessee Department of Education of its internal investigation into the matter

February 2017 — Shelby County Schools hires Dixon Hughes Goodman, a CPA firm headquartered in Charlotte, N.C., to conduct an audit on all high schools

June 1, 2017 — Mackin submits resignation letter to Shelby County Schools