Mill River Union High School, commonly known as Mill River or MRU is a public junior-senior high school in Clarendon, Vermont, in the United States. It is part of the Mill River Unified Union School District. MRUUSD includes the towns of Clarendon, Wallingford, Shrewsbury and Tinmouth. Other towns whose students attend Mill River include Ira, Ludlow, Middletown Springs and Danby. The current principal is Kaelyne Thompson. The current superintendent of the district is Brian Hill.

Clarendon is a town in Rutland County, Vermont, United States. The population was 2,412 at the 2020 census. Clarendon spans U.S. Route 7 and is split by the highway, the Cold River and Mill River, Otter Creek, and the Green Mountains into the hamlets of Clarendon, West Clarendon, East Clarendon, Clarendon Springs, and North Clarendon