Voters will consider a proposed debt exclusion to fund the construction of a new Galvin Middle School at a special election on Saturday. Polling will take place from 7 a.m. to 8 p.m. at the field house behind the high school. The Permanent Building Committee and the Galvin Middle School Building Advisory Committee are jointly hosting a public forum on the project at 7:30 p.m. Wednesday in the Galvin auditorium. The $73.96 million project calls for construction of a new 187,732-square-foot fifth- through eighth-grade middle school on the site of the existing Main Street school. The annual Town Meeting on May 14 authorized the town to borrow $73.27 million for the project, representing the full cost minus $686,400 previously appropriated. The vote was contingent on voter approval of the debt exclusion, or temporary tax increase, to fund the town’s share of the project. The Massachusetts School Building Authority has committed to reimbursing the town for 54.67 percent of eligible costs.