IT DOESN'T take the services of a consultant to figure out that one key to successful growth in a county is a good school system. Without vision, money or strong support, the schools in a rapidly growing county can become a serious liability to more than their No. 1 victims, the students. Deficient schools can also impede efforts to attract the kinds of industries, commercial services, cultural events and amenities that can make a permanent difference. This is why the best resident minds in Northern Virginia's rapidly changing Prince William County are encouraging the ambitious initiatives of their school superintendent, Edward Kelly -- who has some big ideas for the next six years of public education in his growth-strained system.

Mr. Kelly has produced a detailed "six-year plan" that is anything but modest in scope, along with a capital improvements program necessary to make any such plan work. Proposals include longer school days beginning next year, stiffer graduation requirements by 1992, more autonomy for school principals and the construction of 14 schools to accommodate increases in enrollment. Mr. Kelly says approval of the plan would change the school system from "very good" to "outstanding." By the end of the 1993-94 school year, the superintendent envisions that 90 percent of students from kindergarten through senior year in high school would be at or above their grade levels in reading and math. All students would be passing a literacy test that the state will begin administering in 1990; and blacks and whites would be performing equally well on standardized tests. In addition, Scholastic Aptitude Test scores, national merit semifinalist results and scores on advanced placement exams would improve.

That's some tall order for a system that already has grown so swiftly that 100 trailers are being used as classrooms, along with other makeshift facilities. This year alone, enrollment increased by 1,281 students to a total of 39,100. Last month, voters took an important step when they approved $44.89 million in bonds for the construction of two elementary schools, an addition to an existing elementary school, a middle school and a seventh high school. But with expected annual enrollment increases of 1,300 a year, Mr. Kelly is calling for the construction of 109 elementary schools, three middle schools and an eighth high school, all scheduled to open between 1991 and 1995.

Members of the school board may be understandably wary of leaping into massive projects. But they should be just as concerned that failure to act boldly now could make a mess of public education in a county that has the potential to thrive -- or deteriorate -- in the immediate years ahead