Prince William County School Superintendent Richard W. Johnson unveiled a plan yesterday that calls for replacing seven county schools at a cost of almost $45 million in an effort to bring school locations into better alignment with changing growth patterns.

Instead of allocating large amounts of money during the next five years to renovate and expand five elementary schools and two middle schools, the school system should build new facilities at new locations, Johnson said in releasing a capital improvement program for next year.

Several county schools face serious overcrowding or are in areas far from the students they serve, forcing bus trips of 20 miles or more for a large number of Prince William students.

Johnson's plan for accommodating the rapid population growth in Prince William, which has the third largest school system in Virginia, is subject to approval by the School Board and Board of Supervisors.

Johnson said the alternative to school replacement was "a Band-Aid approach" of renovating old facilities ill-suited to the county's needs.

"Do we always want to be behind the eight ball, or can we sometimes be racking the balls?" Johnson said in characterizing his plans.

The new schools would be built with money granted by the Board of Supervisors, which would then take ownership of the old properties. Many of the properties are considered attractive as sites for other county facilities, or as land to be sold to developers.

Yesterday's proposal for replacing the schools, added to previously announced plans to build a seventh high school and two elementary schools, gives the county a total of more than $70 million in new school construction slated for the next five years.

The proposed elementary schools would be built at a cost of $4.45 million each, and the middle schools at a cost of $10.99 million each. The new high school would cost $22.5 million, and will be placed before voters in a bond referendum in March 1987.

Johnson's plan for replacing the schools drew early praise yesterday from Supervisor G. Richard Pfitzner.

"I applaud any approach that looks comprehensively at how the whole school system is growing," he said. " . . . We have all recognized for some time that there are some of our schools that are basically illogically located."

This problem has been exacerbated recently as western Prince William County has joined in the rapid expansion that has been a fixture in eastern areas for 20 years.

"You know and I know that development is coming to western Prince William County," Johnson said. " . . . You've got to spend where the money [is needed] in the next 10 to 15 years."

The facilities Johnson proposed to replace are Bennett, Dumfries, Gainesville, Occoquan and Yorkshire elementary schools and the Marsteller and Saunders middle schools.