A deal to sell the town’s 138-year-old Colburn School has been sealed and Brockton-based Coffman Realty has agreed to pay $350,000 for the historic building being stored next to the new Westwood Public Library. Once construction is complete on the new library, the school — which is perched in the back of the parking lot — will be inched to its new home at the site of the old library, which has been demolished. Purchasers will pay the town close to $90,000 for completed site work. Coffman plans to convert the building’s first floor into a bank and its second floor into six residential units. The 200-ton, three-story building, an example of Second Empire Victorian architecture, was moved in December 2010 to make way for the new library. The school is named for Warren Colburn, a descendant of the town’s earliest settlers and a famed mathematician. Before the building became vacant it was used to house the town’s public school administration.