If Prince William County school administrators have their way, high school students in Virginia's fourth-largest school district will have longer days and be asked to meet tougher standards before they're handed their diplomas.

In a three-volume "Six-Year Plan" and accompanying Capital Improvements Program, Superintendent Edward L. Kelly yesterday revealed long-range goals for the school system. Included are longer school days next year, stiffer graduation requirements by 1992, increased autonomy for school principals and the construction of 14 schools to handle increases in the student population in the early 1990s.

Kelly will formally present the package -- described as the most detailed long-range planning document county school officials have ever produced -- to the seven-member school board tomorrow evening. The first phases of the plan will come before the board for approval early next year.

If approved, the plan would transform the school system from one that is "very good" into one that is "outstanding," according to the superintendent, who took over in July.

The most ambitious and expensive item in the instructional plan would be the addition next fall of a seventh period to the current six-period high school day. School officials estimate the cost of hiring more teachers to implement the expansion at $3.28 million in fiscal 1989.

The increase of graduation requirements from 20 units to 22 units would be phased in over the next five years, with a corresponding increase from 22 units to 24 units for an honors diploma.

Site-based management, which is the practice of giving principals more autonomy over personnel and budgetary concerns, also would be phased in gradually, according to Kelly. Some training of principals would begin this school year. Under the Six-Year Plan, all county school principals would have greater authority by the beginning of the 1991-92 school year.

In response to the rapid growth of the student population, the Capital Improvements Program presented by Kelly calls for the construction of 10 elementary schools, three middle schools, and an eighth high school, all scheduled to open between 1991 and 1995. County projections anticipate annual increases in enrollment of nearly 1,300 over the next six years.

There are currently 39,100 students enrolled in county schools, and 100 trailers are already in use at crowded schools.

Last month voters approved $44.89 million in bonds for the construction of two elementary schools, an addition to an existing elementary school, a new middle school and a seventh high school, all due to be open by September 1991.

The Six-Year Plan also laid out goals that Kelly acknowledged were ambitious but that he said are not "unattainable." By the end of the 1993-94 school year, he said: Ninety percent of students in kindergarten through grade 12 will be at or above their grade levels in reading and math. All students will pass the so-called literacy passport test to be administered by the state starting in 1990. Blacks and whites will perform equally well on standardized tests. Academic performance as measured by Scholastic Aptitude Test scores, numbers of merit semifinalists and scores on Advanced Placement exams will improve.