A delegation of Fairfax County high school students put aside their books and lessons yesterday to try to persuade Chairman John F. Herrity of the county Board of Supervisors that their teachers should get a bigger pay increase.

But after meeting with the students in his office and being shown a petition in support of the teachers signed by 1,100 of Robinson High School's 1,500 students, Herrity held his ground on the pay question.

"I'm doing what I think is right," Herrity told the students. "If I'm not elected after doing what I think is right, let the chips fall where they may. I could go out of here and make a lot more money."

Nor did Herrity offer any hope that his colleagues on the Board of Supervisors would be persuaded by the students. "The board has made its decision," Herrity said, "and I don't think it will change its decision."

Last week Herrity and other supervisors approved a budget for fiscal 1980 that would give the public school teachers a 5.15 percent raise-the same that was approved for other county employes.

The Fairfax Education Association, which represents most of the teachers, has called for a 9.4 percent raise. The FEA says any smaller amount would create an even wider gap between salaries and the rapidly rising cost of living.

During the meeting with Herrity, Bruce Cohen, a senior at J. E. B. Stuart High School, appealed to Herrity, saying, "The teachers just can't afford to live in Fairfax County on this (5.15 percent increase)."

"We have not paid the highest cost of living in the metropolitan area," Herrity replied, "but we haven't paid the lowest."

Two robinson students, Ginger Gering, a junior, and Marla Duffield, a senior, complained that Herrity, during a visit to their school several weeks ago, referred to teachers as "part-time county employes."

Herrity denied characterizing the teachers that way. In an interview later, he said, "I cannot recall making that statement."

However, Marilyn Stafford, in whose history class Herrity spoke, said she heard him make the statement from her seat in the back of the room. "I was quite angry and resentful," she said. "I work many hours a day-an average of three extra hours a day and a total of eight extra hours on a weekend. We all do; this is part of our profession."

In protest against the 5.15 percent raise, the teachers have decided to work only for those hours-7:30 a.m. to 3 p.m.-for which they are paid and only on the 200 days in their contract.

This decision already has had an impact on outside activities for which, in the past, teachers have volunteered to do additional work at no extra pay Students from Chantilly High School asked Herrity to attend a May 12 dance at the school as a substitute for a teacher who ordinarily would have attended-and he accepted the invitation.