Many of the 400 seniors at Fairfax County's J.E.B. Stuart High School were stunned yesterday when the school suddenly canceled a long-planned reception for the graduating class.

The reception was to be held Sunday, but teachers announced yesterday they wouldn't show up for it.

The cancellation was one of the first tangible results of an unprecedented job action called this week by the Fairfax Education Association, which represents most of the suburban county's 7,000 teachers. The association members, angry over the county's refusal to grant them a 9.4 percent pay raise, voted overwhelmingly to end any volunteer work for the schools.

Both protest leaders and school administrators agreed yesterday that it was difficult for them to assess the job action's impact. But Robert Hicks, president of the protesting teacher group predicted that numerous extra-curricular activities will be curtailed because of the action.

"I think it's awful," complained 18-year-old Stuart High senior Kim Phelps yesterday after the school abandoned its plans for the senior reception. "The kids are always the victims when something like this happens," said Terrance Shea, a Stuart senior and president of the student government there.

At George C. Marshall High School near Tysons Corner students were more sympathetic to the teacher protest. A student government group there voted to cancel a Saturday night dance that would have benefited the American Heart Association, saying it didn't want to embarrass the four-teachers who had agreed to chaperone the dance.

"The teachers were in a conflict," said Grace Taylor, the president of Student Cooperative Association at Marshall. "They wanted to help us (the students). But they were being pressured by other teachers."

"We didn't want to take advantage of them," said the 17-year-old Taylor, who said the student group plans to donate the $175 they would have paid the band to the American Heart Association.

"We don't want the students to be hurt," said Laurie Williams, a teacher at Marshall. "But, we have had it."

Williams said she has cancelled a field trip planned for one of her classes at the Kennedy Center next month as part of the protest.

"I feel bad for the kids because they are missing out on something that they have been looking forward to," said Pam Martinov, a government teacher at Stuart. "On the other hand, if the teachers are going to show that they really are serious about the work-to-the-rule (action), they have to do it. It (the senior reception) was the first issue to come up and they've got to stcik to their guns."

Richard W. Johnson, Stuart principal, said the senior reception has been a tradition at the 20-year-old school located at 3301 Peace Valley La., near Baileys Crossroads. He said teachers voluntarily helped to prepare the food and served as hosts for the reception. "We can't sponsor the activity without the staff," he said.

County School spokesman George Hamel said yesterday he wasn't aware of the cancellations. "It's too bad these things have to be canceled. . . We would hope the teachers wouldn't do anything that would have an adverse effect on the academic program."

In voting to perform only work they are paid for, the teachers were advised if they are instructed by administrators to perform extra tasks, they should do so under protest that will enable them to avoid being accused of insubordination and subject to dismissal proceedings, protest leader said.