ROCKLAND, Maine — The town of Rockport is suing the Camden-based school district where it sends students after it was discovered that the district was incorrectly calculating the town’s yearly education payments for nearly a decade.

The error meant Rockport overpaid for its share of annual funding for Maine School Administrative District 28, according to the lawsuit, which claims that Camden underpaid to fund the towns’ joint school system.

Rockport filed the lawsuit on Friday in Knox County Court against MSAD 28 and the town of Camden. Rockport is seeking reimbursement or credit for its overpayments.

“By applying an incorrect assessment formula […] MSAD 28 overassessed and overcharged Rockport significant amounts to the benefit of MSAD 28 and Camden,” the lawsuit states.

MSAD 28 provides public education for Camden and Rockport students in kindergarten through eighth grade. These students attend Camden Hills Regional High School, which is operated by the Five Towns Community School District.

In Maine, the majority of school districts generate a large portion of funding from local property taxes, which are based on property values.

For at least the past nine years, MSAD 28 has been using the wrong education funding formula to generate revenue from the towns in its district, according to the lawsuit. Instead of billing each town based on its property valuations, it has calculated payments based on the percentage of students each town has in the district.

As of the most recent school year, Rockport students made up 47 percent of the district’s student population. However, Rockport’s overall property valuation made up just 42-percent of the district’s total valuation. This error resulted in Rockport overpaying by about $450,000 for the 2019-20 school year and Camden underpaying by the same amount, according to the lawsuit.

MSAD 28 realized the error earlier this year after the district’s new business manager found discrepancies between Excel spreadsheets used by the business office to determine each town’s annual payment and his own calculations on what the amounts should be based on the state’s education-funding formula.

The error was found in time to adjust the payments for the 2020-2021 school year, according to a statement from the school district published in the Courier-Gazette earlier this summer.

However, “due to changing state funding formulas, lack of historical data and natural workplace turnover, it has not been possible to determine how far back in time an incorrect formula was used,” the statement said.