An arbitrator enlisted to settle a dispute between Arundel and Regional School Unit 21 and Thornton Academy has sided with the private school, ordering the school district to pay the school tuition for any students sent from Arundel to the Middle School of the Kennebunks.

Thornton Academy said the ruling means RSU 21 will owe the school more than $300,000 if all 19 students who previously expressed interest in going to the Middle School of the Kennebunks end up enrolling in the public school.

The dispute arose from a contract Arundel signed in 2005 with Thornton Academy which said that “all” middle school students in the town would to be sent to the Saco private school. Thornton Academy spent about $3 million to build a middle school based on the contract and Arundel students make up the vast majority of students in Thornton Academy’s middle school program.

After Arundel joined the RSU under the state’s consolidation law, the school district explored sending middle school students to the Middle School of the Kennebunks. School district officials said the Middle School of the Kennebunks has the capacity for more students and contend district taxpayers would save if the students were sent to a school in the district instead of paying tuition to the private school.

The RSU has also decided to buy out the rest of the contract between Arundel and Thornton Academy after the upcoming school year at a cost of $1.2 million, although that decision still must be ratified by voters. Under state law, students would still have the choice to go to Thornton Academy, with the district paying tuition, or to attend the Kennebunk public school.

RSU 21 this spring began soliciting interest from parents who might want to send their students to the Middle School of the Kennebunks, and Thornton Academy cried foul, citing the language referring to “all” students in the contract. RSU 21 said that other language, requiring Arundel to pay the school tuition only for the students who enroll at Thornton Academy, suggested that all students in the town were not required to go to the private school.

In early July, arbitrator Patrick Coughlan ordered RSU to stop enrolling Arundel students at the Middle School of the Kennebunks until he ruled on the dispute.

Coughlan’s ruling issued today found that Arundel was obligated under the contact to send all its 6th-, 7th- and 8th-graders to Thornton Academy and the RSU would violate the contract by enrolling students from the town at the Middle School of the Kennebunks. He also said that the district will have to pay tuition for any students from Arundel who attend the Middle School of the Kennebunks until the contract is ended and, since students who opt to attend Thornton Academy in 6th grade would likely stay there the next two years, RSU 21 will have to pay 75 percent tuition for those students who stay at the Middle School of the Kennebunk for the following years.

Thornton Academy said today it will be owed $315,000 over the next three years under that formula if all 19 students who expressed an interest in attending the Middle School of the Kennebunks enroll in that school and stay through their middle school years.