Spending guidelines set by the Fairfax County Board probably will not permit school construction to keep pace with the county's galloping growth, School Superintendent Robert R. Spillane told a weekend conference.

The fiscal constraints are so severe that they may require the county to consider "creative solutions" for coping with growth, such as year-round classes in selected schools or flexible scheduling, he said. Spillane did not endorse year-round classes, but he did call for asking Fairfax parents whether they would endorse the idea.

Spillane's remarks, which could revive tensions between the School Board and supervisors over the county's education needs, came in a speech Friday night at the annual School Board retreat in Fredericksburg, Va.

In summarizing his first year as head of Fairfax schools, the former Boston school superintendent offered a broad blueprint for the future of Fairfax, which has the nation's 10th largest school system.

School Board members generally praised Spillane's speech, particularly his proposal to allow principals more freedom to run their schools and to impose fewer across-the-county rules. "It is the most exciting presentation I've heard . . . a terrific presentation," said Kohann Whitney, who represents the Centreville area, which has some of the county's fastest growing neighborhoods.

Even if the county could afford the new construction it needs, Spillane said, he has serious doubts that the construction industry could meet the demand. "We're going to be running just to stay even -- which is a little behind -- for the next five years," Spillane said.

Planners predict that school enrollment, forecast at 129,000 next year, will rise to 145,000 by 1995 and 170,000 by 2015 -- most of it in the elementary grades, he said. That will require 24 additional buildings by 1995, and 15 more by 2015, Spillane said.

Even more space will be required if the county yields to pressure from working parents to open more extended day care centers in schools, or if the School Board institutes all-day kindergarten sessions as some propose, he said.

The supervisors currently allow no more than $80 million a year, or $400 million over five years, to be spent on all county bond-funded construction, including schools and roads. Half of that -- $200 million over the next five years -- is being spent on new school construction, and it is "unlikely this will be sufficient to meet demand for new school buildings," Spillane said.

Spillane's remarks raised the possibility of reopening a rift with the county supervisors, who imposed the construction limits in order to preserve the county's prized AAA bond rating, which enables the sale of bonds at low interest rates.

Several months ago, County Executive J. Hamilton Lambert touched off a controversy with the school system when he sent a memorandum to Spillane, warning that the School Board's 1987-91 construction plan could violate those bond guidelines. The tiff was settled without the School Board changing its plan, but not before charges flew that the supervisors were trying to allow rapid housing growth without supplying needed services.

Because of construction limits, the county must think creatively about ways to cope with rapid growth, Spillane said.

A survey in 1974 found that 59 percent of parents were willing to have their children attend year-round school, he said, adding: "We may need to consider . . . in some selected areas of our system . . . a look at extended school years."

Los Angeles is planning for a year-round classroom schedule to make room for increases in its school enrollment. Prince William County tried year-round schools in the 1970s but returned to a nine-month schedule in 1979 after teachers, parents and administrators complained. Year-round programs stagger attendance by giving students the equivalent of a summer vacation at various times during the year.

In other remarks, Spillane said:

*The county should give its principals freer rein to run their schools and impose less bureaucracy from above. That could mean more variety among schools, he said. If the county allows principals more room to experiment, it also must hold them accountable for increased student achievement -- and must be willing to risk public anger or grievance procedures by getting rid of bad administrators, he added.

*The county should reach a multiyear contract with the county's 7,000 teachers this year, averting the annual "pageantry" of salary talks and last-minute agreements. A commission Spillane appointed to study a new salary schedule for teachers, and other ways to improve the profession, is scheduled to issue its report this month, and the panel's recommendations are expected to form the basis of Spillane's salary offer.

Although Virginia law prohibits collective bargaining for government employes, the two teacher associations in Fairfax were so disgruntled by Spillane's salary package that they have been staging a job slowdown since December.