School officials have reached an agreement with the local teachers union on a three-year contract that will take effect in September. School Committee chairwoman Kirsi Allison-Ampe and Siobhan Foley, vice president of the Arlington Education Association, announced the agreement after the union voted to ratify the contract on June 18. The agreement, which includes an overall pay increase of 2.5 percent, comes about six months after the town shifted its public employee health care coverage to the state’s Group Insurance Commission. In their joint announcement, Allison-Ampe and Foley said the new contract was made possible by a property tax increase approved by Arlington voters last year as well as the change in health care coverage, which is expected to save the town several million dollars in the coming years.