Mary Hynes considers herself one of Key Elementary School's biggest boosters. Three of her children attended the Arlington school. She served as PTA president and has high praise for the school's teachers and its rich mix of foreign-born pupils.

But this year, when it came time for her daughter Shannon to begin the first grade, Hynes broke the family tradition and transferred the child to Taylor, where an overwhelming majority of pupils speak English as their first and only language.

She says she made the decision when she saw the list of Key pupils who would have been in Shannon's first grade class. "I just looked down the list and everyone had a Spanish surname," she said.

Hynes' reaction reflects that of a growing number of middle-class Anglo parents in Arlington who have expressed concern that their children's educational and social experience in school is being adversely affected by concentrated numbers of newly arrived immigrant children.

Some Anglo parents have complained that typically American social traditions suffer in a setting where they are new to the student body. They complain of not being able to maintain Girl and Boy Scout troops and about foreign languages spoken on the playground. "There's nobody to play with," explained Judy Buchholz, a mother of three Key pupils.

Without exception, the pupils and six teachers interviewed at random during visits to Key agreed that the large immigrant population affected the instructional and social atmosphere at Key.

But, in contrast to some of the parents, the pupils and teachers described the effect only in positive ways, including the social benefits gained through knowing people of other cultures. In addition, the teachers argued that foreign-born pupils do not have a negative impact on classes because they have been able to adapt their lessons to accommodate them.

In the 1986-87 school year, Arlington had three of the 12 Washington area schools in which immigrants made up more than 50 percent of the student population, according to figures compiled recently by The Washington Post. Ten years ago there was none.

Arlington students are among the most ethnically diverse in the area, with 52 percent of its students being white, 18.4 percent Hispanic, 16.4 percent black and 13 percent Asian.

About 15 percent of Arlington's 14,258 students are enrolled in full- or part-time English-language classes. At Key, Barrett and Glencarlyn elementary schools, 40 percent of the pupils are in these courses. At Key, another 30 to 40 percent are immigrants who have learned English well enough to attend regular classes full time. About 35 percent of the non-English-speaking pupils are in preschool and first grade.

Of Key's 573 pupils, Spanish is the native language of 272 pupils, and English is the first language of 193. The other pupils speak one of 12 other languages as a mother tongue.

Last year, responding in part to parental concerns, Arlington Superintendent Arthur W. Gosling appointed a citizens committee to investigate the impact of these pupils on county schools. One goal of any solution, said school officials, is to stop the small but growing exodus of Anglo middle-class children from schools with large foreign-born student bodies.

This month, after six months of debate and investigation, the committee delivered a report that outlined alternatives, ranging from districtwide busing of all students to achieve linguistic desegregation to relocating immigrant students in special centers until they master English.

But John Crowder, the school official in charge of the committee, said the 33-member group could find no studies or hard evidence to support the perception by some parents that the language deficiencies of these students negatively affect the educational or social milieu at a school. Rather, he said, "the impact is in the eyes of the beholder."

Gosling contends that that perception alone is enough to merit School Board action on the matter. "It is important that we are able to sustain the support of the broad-based community," he said. The concerns include "a combination of {academic} performance, people's attitudes and people's views of what is a reasonable expectation of a school."

Last summer, members of the affluent Lyon Village community, part of which is contained within Key's attendance boundary, surveyed parents about their perceptions of neighborhood schools. More than 50 percent of the respondents described the school's "least desirable" qualities as "too many minorities," "low test scores," "inactive parents" and "overcrowded." Also, more than 50 percent said its "most desirable" characteristics included "good teachers" and "good facilities."

The survey was collected from the parents of 274 youngsters, 81 of whom were elementary or preschool children living in the Key boundary area.

School district officials said there are no reliable figures on the number of pupils who have been withdrawn from Key or the two other schools where foreign-born pupils dominate. But, they said, the numbers were disproportionately large among Anglo pupils when compared with withdrawals at other schools.

"It has been true that you have more instructional transfers out" of these schools, said School Board member Dorothy H. Stambaugh. "The most consistent reason that people transfer out is the instructional program . . . and that can relate to the number of {immigrant} students at the school."

Lori Maes, Key's PTA president and one of two Hispanics on the superintendent's committee, said that the Hispanic community supports Key and that the School Board should devote more resources to helping immigrant children learn English if they are worried about their impact on other pupils.

"These children are going to be around, and we have to educate them," she said.

Key, at 2300 Key Blvd., is in a largely stable, middle-class community next to the Arlington courthouse. Within its enrollment borders sits the low-income Lee Gardens apartment complex, where most of the school's Hispanic children live.

Pupils are placed in regular classes only when they meet certain school district criteria. Under these rules, pupils can be mainstreamed into a class appropriate to their age even if their reading ability is as much as two years behind their grade level. The belief is that once they reach that level, they can catch up more quickly if they are in regular classes.

Teachers, for the most part, argued that they were able to adapt their lesson plans so that neither the immigrant pupils nor the native-English speakers suffered.

"There are difficulties in science and health, for instance, where they need to get information from a book," Karen Spees, a third grade teacher, said of some of her immigrant pupils. "There are cases where vocabulary can slow down reading, but by {having the pupils} talk back and forth, what they don't get from reading they get in class discussion."

Said sixth grade teacher Joan Myers, "You've got to go a little bit slower. We do a lot of writing . . . . I make a point of making sure {immigrant pupils} participate. I fuss at them, and as far as they're concerned, every one of them is up on top of it or this lady gets ugly." Five of Myers' 22 pupils are native speakers of English.

Teacher Alan Tonelson has turned each of his sixth graders into a mentor for other pupils in the class. Many of the American-born children help immigrant pupils with reading. Some immigrant pupils take leadership roles in science and sports.

Tonelson said the device encourages strong self-esteem and enables the mentor pupil to work on verbal and organizational skills through developing one-on-one lesson plans.

In each class at Key, pupils are divided into three groups, based on their reading ability. In the lower grades, gifted pupils who do not choose the school's English-Spanish immersion program share the same classroom with other pupils their age. In higher grades, the gifted students and the high achievers attend a special class that combines the best students from two grade levels.

Pupils and teachers said they see many benefits to the cultural mixture at Key, and they note that the school is virtually free of cultural-based cliques, except among pupils who are just beginning to learn English.

"I don't see prejudice in this room," said Spees. "I don't see people making fun of people's names. It sort of renews my trust in people."

The children seem oblivious to the cultural concerns of their parents. Many take great pride in the school and describe their best friends as people from other countries.

"We don't have that many people from America, so we can learn new stuff about other countries," said Hynes' son Brendan, whose two best friends last year were Chinese and Vietnamese. "They always made me laugh and had fun things for me to do."

"Some of my friends ask me, 'What do you do at your school? how do you communicate?' " said student president Yael Latt, 11. "They say that their schools are better, but I don't think that's true. I think this school is better because it lets all sorts of people in."