Lincolnshire County Council has reacted strongly against West Grantham Academy Trust's decision to consult on the closure of Charles Reed High School.
The Trust has gone ahead with the move despite concerns raised by the local authority, which is responsible for ensuring there are enough places available for children to attend school.
Parents have been told in a letter that reasons for the decision inculde the falling number eligible for secondary education in the area in the next five years and the impact of changes to funding leading to reduced income for schools.
But the Conservative led county council says the proposal to close the school will be detrimental to education in the Grantham and Corby Glen area.
Patricia Bradwell, executive member for children's services, said: "This is a decision that has been taken by the academy in consultation with the Department for Education.
"It is not a county council decision and we are disappointed that the Academy Trust is taking this action.
"To place Charles Read pupils at St Hughs would result in young people travelling vast distances to school and we do not think that parents would want this.
"The proposals would also significantly increase transport costs which would have to be met by the county council."