KENNEBUNK — The RSU 21 School Board on Monday voted unanimously to buy out the remaining five years of its contract with Thornton Academy Middle School.

RSU officials say the buyout, which would go into effect June 30, 2011, would save at least $1 million. They say the savings could be greater if more students opt to stay within the district.

The contract between the private Thornton Academy and Arundel was negotiated in 2004, before Arundel joined Kennebunk and Kennebunkport to form RSU 21.

Thornton Academy spend nearly $3 million four years ago on a building to house the middle school. Only 18 of the middle school’s 163 students were not from Arundel.

RSU 21 officials say that the Middle School of the Kennebunks has the capacity to absorb all of Arundel’s middle school students. The board also voted Monday to accept as many as 15 Arundel students in each of the six, seventh and eighth grades in the coming academic year.