Salaries for Prince William County's 2,450 teachers would increase by an average of 7.5 percent in the next school year and by 8.5 percent the following year under an agreement between the county teachers association and school administrators.

The agreement, reached last week, must be voted on by both the association's membership and the county School Board.

If the pay plan is approved, the average teacher's salary would rise from $29,563 to about $31,800, and the pay of starting teachers from $21,000 to $22,500.

In neighboring Fairfax County, administrators and teachers recently reached tentative agreement to raise salaries by an average of 8.8 percent in the next school year, bringing the average teacher's salary there, now $33,282, to about $36,210. Beginning teachers would be paid $23,100.

While the gap between the average salaries in the two counties would widen, the difference between salaries of beginning teachers would diminish from $1,000 now to $600 in the next school year. That is significant for Prince William, which must compete for new teachers to serve its growing student population.

School Superintendent Edward L. Kelly said the higher numbers in Fairfax may be "somewhat misleading."

"Anytime you have a pay-for-performance plan {which Fairfax instituted last year}, it will be costly. How Fairfax distributes its money is different from ours," Kelly said.

Prince William has no pay-for-performance plan.

Cameron Yow, executive director of the Prince William Education Association, which represents 1,560 of the county's teachers, said he was "pleased with the {negotiating} process and the product." The proposal would add about $5.5 million to the $88.4 million the county spends on instructional salaries and benefits, according to school spokeswoman Kristy Larson.

In addition to higher pay for first-year teachers, the proposal would mean increases of almost 12.5 percent for teachers with three years of experience. Teachers on the upper end of the scale will get smaller percentage increases.

"It's difficult to balance the needs of a growing system -- the money you need for buildings and programs, realizing that they don't mean anything if you don't have qualified teachers," Kelly said. "Of course, we'll have to monitor other areas to see that we stay competitive." The 39,100-student system is Virginia's fourth largest. Enrollments grew by 3.2 percent this year.

The teachers association's leadership will recommend that members accept the proposed salary increase, and a vote will be taken by mid-February, Yow said. Kelly is expected to submit his budget, including the increase, to the School Board by early March.

Yow said he was extremely pleased with the half dozen meetings between administrators and association representatives that yielded the salary increase proposal.

"It's the first time in recent history that we've had any productive sessions," Yow said, referring to the relationship between the association and former superintendent Richard W. Johnson.

According to Yow and Jeff Bloomer, who was acting superintendent from January until July 1987, the administration previously made salary increase proposals unilaterally.

"In the past, we had no idea what the proposal would be until it appeared in the budget," Yow said. "There was no responsiveness on the part of the administration."