The Alexandria School Board approved the first major reorganization of the city's secondary schools in eight years last night, voting to have 10th grade students attend T.C. Williams High School, which already houses 11th and 12th grade classes. Two three-year middle schools also will be created.

Reorganization to make better use of school facilities had been called "the only solution" to continually declining enrollment and escalating costs by School Superintendent John C. Bristol.

Bristol initially favored a plan by which two conventional four-year high schools would have replaced the current 6-2-2-2 system under which students change schools every two years after completing the sixth grade and all 11th and 12th graders attend T.C. Williams.

But the two-high-school plan was opposed by many parents who argued that preservation of T.C. Williams as the only senior high would least disrupt current educational patterns. Advocates of maintaining the current system also argued that since it was instituted as a desegregation move in 1971, discipline problems have been greatly reduced, there have been substantial gains in test scores and morale at the schools, and the 11th and 12th graders have had a greater variety of course offerings.

Both sides in the controversy cited the need to attract more families with children to the city to help offset declining enrollment.

The reorganization plan that will go into effect next fall was approved last night on a 5-to-4 vote and was proposed by board member Judith Feaver. The majority viewed it as desirable because it would provide relatively little disruption of current patterns while eliminating a large number of school buildings.

Francis Hammond and George Washington Middle Schools, which now house the 9th and 10th grades, will be converted to 7th, 8th and 9th grade use.

The school system will close and recommend selling the Adams and Parker-Gray schools, which now house the 7th and 8th grades, the Secondary Schools Occupational Center and the school administration building in downtown Alexandria. Minnie Howard School, which now has 9th and 10th grades, will be used for administrative offices and house the occupational center.

When the city's 834 10th graders shift to T.C. Williams in the fall, the school's enrollment will swell to about 2,500, feeling it to its designed capacity. Superintendent Bristol said conditions at the school will be "tight" for two or three years until enrollment shrinks.

Bristol called the approved plan feasible and said he thinks he can make it work well. The 6-3-3 system "has worked in many systems," he said, "there is no reason why it can't work here."

Cost of making renovations and adding an auxiliary gym at T.C. Williams to accommodate the additional students there and of renovating Hammond and George Washington middle schools is estimated at $4.6 million. The cost will be included in the 1979-80 school budget to be discussed at the board's meeting next Wednesday. The budget must be submitted by April 1 to the City Council, which has the final say over spending.

Consolidation of school facilities was prompted by a 30 percent enrollment decline from a peak of 17,200 in 1972 to 11,500 today. A decline of 3 percent per year is predicted over the next three years.

Voting with Feaver to add the 10th grade to T.C. Williams were board members Claudia Waller, Lou Cook, Wilford Smith and Alison May. Voting against were John Peterson, Shirley Tyler, Michael Mulroney and William Euille.