Mentally handicapped youngsters and nonhandicapped children don't mix well in the classroom, according to a survey of teachers in a Fairfax County school district.

While the practice of placing handicapped youngsters in conventional classes, called mainstreaming, may be helpful to the handicapped child, it is extremely disruptive to others in the class, the teachers said.

Federal regulations on special education recommend, however, that the two groups be taught together whenever possible.

Intermediate-school teachers were the most critical of mainstreaming; 79 percent of the teachers polled said they felt the practice had a negative impact on their classes and recommended revisions in the program. About two-thirds of the elementary and high school teachers said the program should be changed.

The problems of handicapped students evoked the strongest response from the 208 teachers polled. The survey, released last week, was commissioned and funded by school board member Gerald Fill, who represents the Mount Vernon district. About one-fourth of the teachers in the district responded.

Fill paid a Fairfax teacher $500 to develop and conduct the survey.

"The results were broadly representative of Mount Vernon district teacher views, according to survey specialists," Fill said in the poll analysis. The Mount Vernon district covers the eastern end of Fairfax County, one of the most economically diverse areas of the school system.

The poll was the first comprehensive study of teacher attitudes toward the county schools' instructional programs, according to Fill.

Teachers gave the instructional programs a "B" rating overall; 59 percent of the teachers polled rated the quality of instruction "very good." The system received an "outstanding" rating from 17 percent of the teachers; 19 percent said the programs were "good;" 4 percent rated them "fair" and 1 percent ranked them "poor."

Teachers rated the factors they considered most supportive of their classroom teaching, giving the highest marks to administrative support. The lowest ratings went to teachers' aides, curriculum specialists, guidance counselors and clerical help.

When reacting to student discipline problems, response from the teachers varied significantly in relation to student age groups. One-third of the elementary school teachers said discipline problems are disruptive to the education process; at the high school level, 9 percent of the teachers reported disruptive discipline problems.

Overall, 52 percent of the teachers said student discipline was moderately disruptive in the classroom; 20 percent said they found discipline problems quite disruptive.

When it came to identifying the factors that caused the most stress for teachers in the classroom, students topped the list.

Almost three-fourths of the teachers said they had encountered occupational stress in the classroom, and most recommended a countywide, school-system program for helping teachers cope with stress.

Fill said he has distributed the survey results to officials throughout the Fairfax school system with the hope that they will consider teacher viewpoints when making school decisions.

He originally commissioned the survey as a trial of a proposal, which he plans to introduce at tonight's school board meeting, to establish a fellowship program within the school system which would pay several teachers to do various types of research for the system and the school board.