BATH, Maine — West Bath is suing Regional School Unit 1 to recover $1.9 million the town believes it overpaid in the first four years of the school district’s existence.

West Bath’s lawsuit, filed by attorney Sally Daggett in October in Sagadahoc County Superior Court, claims it should have been assessed a total of $8.2 million in fiscal years 2008, 2009, 2010 and 2011, but instead paid $10.1 million.

The suit claims RSU 1 owes West Bath $1.9 million, plus interest. The West Bath Board of Selectmen voted unanimously Oct. 9 to take legal action, West Bath Town Administrator Jonathan Davis said on Friday.

The town sought mediation, but RSU 1 “preferred the structure of a lawsuit,” Davis explained. “We understood, so we moved forward with [the process] in that way.”

Timothy Harkins, chairman of the RSU 1 board, said the school district’s attorney, Melissa Hewey, recommended against mediation.

Davis did not know when the matter would go to court.

A statement released by RSU 1 Friday said the School Board was “disheartened” by the lawsuit.

The board “adopted the [cost-sharing] formula that is now being challenged upon the advice of an attorney who represented West Bath, and it has at all times been completely open and above board about how cost sharing was being calculated,” according to the statement. “The formula was discussed at public meetings at which West Bath representatives were present and participated and was set forth clearly when costs were assessed.

“It is regrettable that West Bath did not raise this issue during any of the past five budget seasons so that it could be addressed in a timely fashion,” the statement added, “and it is even more regrettable that having failed to object, and indeed having participated in the process for all those years, West Bath now pursues a recovery of such a magnitude that would ultimately burden the RSU and its member communities with costs that would have a negative impact on the quality of the education being provided to all of our students.”

The RSU 1 Board of Directors voted unanimously April 23 to change the cost-sharing formula for the current, fiscal 2013 budget, so that a law that created the school district would apply to its entire local tax calculation.

The local contribution from the five RSU 1 communities — the funds raised through taxes — had been split into two elements: a minimum amount that the state requires, and a portion over and above that amount. State subsidy to the district had been presented on a form based on the essential programs and services model.

Harkins said in April that the form stated that for the district to receive subsidy from the state, “we need to raise a certain dollar amount at the local level. And they break that down by community. They say each community should raise X amount of dollars.”

The other local contribution piece was the additional amount each community must raise, beyond the EPS model. That contribution stipulated a cost-sharing formula based on equal thirds: student population, state valuation of a community and the community’s population in the most recent census.

The board had heard arguments from the public in support of the change, but its decision drew criticism from some municipal officials.

The Bath City Council in May asked Harkins to request that the School Board “immediately begin the process of adopting a new cost-sharing method that is fair and equitable to the taxpayers of all member communities.”

A committee charged with reviewing RSU’s cost-sharing formula recommended Nov. 27 that a per-pupil cost-sharing formula be implemented in Regional School Unit 1 starting with the 2014-2015 school year.

Initially, the committee voted 3-2 to begin the per-pupil formula next year. Bath, Arrowsic and Phippsburg representatives were in favor of that, and West Bath and Woolwich were opposed.

But in a subsequent vote, taken in an attempt to send a more solid recommendation to the RSU 1 Board of Directors, the Arrowsic and Phippsburg representatives reversed their decisions, joining West Bath and Woolwich. The result was a 4-1 decision in support of starting the per-pupil formula in 2014.

The School Board is expected to vote on the per-pupil assessment, which would be the same for each community, at its Monday, Dec. 17, meeting. If the board supports changing the formula, the decision would go to a districtwide referendum.