Fairfax County students will become more numerous, more ethnically diverse and more middle class in the decades to come, according to a report released yesterday.

By 1990, school enrollment in the county is expected to top 136,000, a gain of almost 5,000 students over this year's estimated enrollment, according to Harold L. Hodgkinson, a demographer and senior fellow of the American Council on Education who prepared the report. In addition, the report said, the proportion of minority students has grown from 9 percent in 1977-78 to 21.6 percent in 1986-87, with most of the new students being middle class because of housing costs in the county.

"Fairfax will diversify by ethnicity but not much by class," Hodgkinson said in the report, titled "Fairfax County: Its Educational System in Context." The report was provided free to the county as a spinoff from a staff development contract between the county and the Institute for Educational Leadership, which acted as liaison with Hodgkinson, according to county Superintendent Robert R. Spillane.

The demographic projections have implications for how county schools will be managed, perceived in the community and staffed, the report said.

As its population grows, the school system will need to increase the degree of autonomy given to principals, which presents problems because "good leaders are not always good managers," Hodgkinson said.

"How effectively principals can improve their management and leadership abilities" will determine how well Fairfax succeeds in attaining its goals of improving minority achievement and making teacher pay dependent on performance, he wrote.

The report also noted that schools will face more competition for available county funds as the population over the age of 65 increases from its current 4.37 percent. Hodgkinson suggests that the schools take a lead in providing services, such as a current program that provides senior citizens with subsidized meals at 15 county schools.

Hodgkinson said schools that provide such services are likely to get support when it comes to school bond issues and other areas where taxpayer support is essential. "Older people will have enormous clout in the years to come" as their numbers grow proportionately, he said.

How to staff future schools also will become a concern, according to Hodgkinson. As housing costs in the county have risen, blue-collar workers and "the working poor" have gone elsewhere to live. Already, according to Hodgkinson, many school bus drivers do not live in the county.

However, he does not see a similar problem attracting teachers.