A fourth Catholic high school is being planned for Northern Virginia to serve the Fairfax City and Reston areas and other communities in Fairfax and Prince William counties.

The Fairfax City Council apparently cleared the way for the project last night by refusing to buy an existing school building at 10675 Lee Highway. That refusal will permit the sale of the building to the Roman Catholic Diocese of Arlington, which apparently is the only bidder for the property and wants to begin half a dozen ninth and 10th grade classes there next September.

The building, which once housed Fairfax High School and now is owned and used by George Mason University, can accommodate 1,200 students, and the diocese wants to offer grades nine through 12 there eventually.

"We're so pleased that this kind of site is available in Fairfax City," said diocesan spokeswoman Ellen McClosky. "With the growth in Reston and Fairfax City, as well as to the south in the Dale City area, the school is needed."

She said the school probably would draw students from as far away as Woodbridge and added that interest in Catholic schooling has increased in the growing outer suburbs, while it has held steady in Arlington and Alexandria, where the other Catholic schools are.

The existing schools--Bishop Denis J. O'Connell High in Arlington, which is coeducational, and Bishop Ireton High for boys and St. Mary's Academy for girls, both in Alexandria--serve 2,410 students, with more than half attending O'Connell, which is too full to accept new students. The new school, whose name has not been announced, would be coeducational.

Last night's action by the Fairfax City Council was necessary because terms of the deed to the property required that the city be given first refusal if the university put the building up for sale.

It could not be learned yesterday how much the diocese is offering for the property, which includes the building and 16 acres of land. City officials said it is assessed at $2.8 million and guessed that it would sell for more than $3 million.