Carlos is a thin, neat-looking 7-year-old who teachers say is polite and quiet in class. But even though schools might well want more students like him, he is at the center of a major educational dispute.

For Carlos, whose family moved to Fairfax County last summer from Bolivia, speaks hardly any English. At Belvedere Elementary School, near Falls Church, he spends most of his time learning English in a special class with students who speak a total of six languages at home.

In Arlington, however, students such as Carlos attend bilingual classes, conducted in both Spanish and English, at Key Elementary School. Their teachers, who speak both languages, teach reading and math in Spanish before trying to cover the same concepts in English.

Which method works better? Which children learn more? Which youngsters move faster into regular English language classes?

The answers to these questions are uncertain, but the debate about them is fierce -- in the Washington area and around the country -- as educators, parents, and politicians wrestle with the dilemmas of bilingual education.

Recently the federal Department of Education proposed new rules, requiring schools to teach basic subjects to children in their native language until they have mastered English.

The proposal was hailed by President Carter and major Hispanic organizations as necessary for equal educational opportunity and as an advance for civil rights. But it drew sharp criticism from a wide range of education groups as a major extension of federal control over schools that they charged was educationally unsound and legally questionable.

President-elect Ronald Reagan made no comment on the proposal during the campaign, but his education advisory committee includes several strong critics of bilingual education, among them Gary L. Jones, a member of the Fairfax School Board.

Even before Reagan's election, Congress barred the Education Department from implementing any new rules regarding bilingual education until next June. fThe new administration is likely to change the rules considerably.

But advocates of bilingual education are pressing a federal court case in Texas and, regardless of what happens to the proposed federal rules, bilingual education will surely remain controversial.

The issue seems to grow more heated as the number of students whose English is limited increases. There now are an estimated 3.6 million in U.S. public schools, about three-quarters of them Hispanic, according to the National Center for Education Statistics.

Some 800,000 are in bilingual classes on which the federal government is spending $167 million this year and state and local governments even more.

In the Washington area there are about 10,000 students with a limited command of English who are taking special classes this fall in English as a Second Language (ESL). The number has increased sharply over the last three years with the arrival of immigrants from Southeast Asia and Central and South America. In Arlington, the jurisdiction with the highest proportion of students whose English is limited, they now make up 10 percent of public school enrollment.

About 1,500 of these students are also enrolled in bilingual programs, two-thirds of them in the District of Columbia and the rest in Arlington and Montgomery Counties.

Unlike ESL classes, in which all teaching is done in English that gradually gets more difficult, the bilingual programs are in two languages: English and the language the child speaks at home.

The bilingual programs include Oyster Elementary School, near Washington's Shoreham Hotel, which has an English-speaking and a Spanish speaking teacher in every classroom. In Arlington there are bilingual classes in Spanish and English, and Vietnamese and English in one elementary school and one senior high.

Montgomery County has bilingual classes in Spanish, Vietnamese and Korean. At one Silver Spring high school, Montgomery Blair, there is a range of courses in simplified English and Spanish-English bilingual classes. Students can graduate without taking any regular English-language courses except gym and a few electives.

"I don't think it happens very often," said Joseph S. Vallani, Blair's principal, "but it is possible for a student to graduate here without learning English very well. Next year they will all have to pass the Maryland Functional Reading Test [which is set at an eighth-grade level and given only in English]. But they need their diplomas to get out and get jobs as soon as possible . . . We have to view ourselves as an aid to the student rather than a hindrance."

The area's other large school systems -- Prince George's, Fairfax and Alexandria -- all offer special classes in English as a Second Language (ESL) to students who aren't proficient in English. But the schools give no instruction in students' native languages, and officials are sharply opposed to the idea.

"We are strongly committed to the principle that no student be denied educational opportunities by lack of proficiency in English," said Prince George's school board member Susan B. Bieniasz. "But we feel very strongly that our approach [teaching students only in English] provides . . . the quickest way of preparing them to [use] English in the classroom . . . in dealing with the community, and in dealing with society at large."

"Yes we want mutual respect and recognition of who a child is," said Esther Eisenhower director of the English as a Second Language program in the Fairfax County schools. "But we draw a line. The purpose of a public school is to help the child succeed in American society.

"What good is it for the child to know all about the Mayas and the Incas and the Conquistadors if the doesn't have the skills to succeed in this English-speaking world as it is?" she said. "Some people are upset about the 'linguistic imperialism' of the United States. Well, one of the unifying things that the country has to have is a language."

In Fairfax County, Eisenhower said, there are abut 2,700 students who can't speak English well enough to handle regular school work. They are scattered throughout 163 schools and speak more than 50 different languages. The largest group speak Spanish -- 575 last year -- followed closely by Korean and Vietnamese.

"How can you be fair to all these groups?" Eisenhower said. "How can you find the bilingual teachers? The only way is to teach them all in English." f

In Jane Johnson's classroom at Belvedere Elementary there were 26 children from 10 countries one recent morning. They spoke six languages -- Spanish, Persian, Korean, Vietnamese, Laotian, and Turkish -- but only a smattering of English, which Johnson, with the help of three assistants, was trying to improve.

As Johnson held up a picture of a dish of ice cream, six youngsters clustered around her. "What is this?" she asked. When no one volunteered the right english words, Johnson gave the answer, and then had the group repeat it.

"Do you like ice cream?" she asked. Most of the children called out, "Yes, yes."

"It is very good to eat," Johnson said slowly before moving on to a picture of an apple and going through most of the dialogue again.

When non-English-speaking children first arrive at Belvedere, the lessons in Johnson's room occupy most of their school day. As their English improves, they spend more and more time in regular classes. After a year or two, Johnson said, most children speak English well enough that the special lessons can stop. Generally, she said, the youngest children make the fastest progress.

At Key Elementary School in Arlington the approach is quite different. About 50 Spanish-speaking youngsters attend classes with three Spanish-speaking teachers and aides. The teachers also speak English fluently and use that language with the English-speaking children in their classes.

For the English-speakers there are no lessons in Spanish. The Spanish speakers do get systematic instruction in English, but they are taught reading and math in their native language before covering these subjects in English.

Often children sitting next to each other work on the same assignment in different languages. For example, English-speakers on one visit were copying a sentence in English: "Christopher Columbus discovered America on October 12, 1492." The Spanish-speakers were copying the same sentence in Spanish.

"Our purpose is to get them into English," said teacher Monica Molina. "But it's a waste of time to have these children sit for a year and not learn their own language when they've already learned how to read in Spanish. So we move them ahead in their own language."

However, not all parents agree with this approach. Two years ago the Arlington school system offered a bilingual class in Korean and English at Glencarlyn Elementary. Last year the class was dropped, said its former teacher Insook Won, when fewer than five students signed up.

"The Koreans want their kids to learn English fast," said one parent, "and they don't want anything in school to slow it down. It doesn't mean that we're all assimilation, that we have no roots. But we know you go nowhere in this country keeping your own language. Whatever pain [the children] go through in learning English is worth it because I hope they will speak English better than you Americans, and go out there and win."

Learning the Korean language is "a family matter, not something for the government," the parent said. He said his children spend Saturday mornings at a private Korean language school that is similar to the Hebrew and Chinese schools that those groups have run for decades.

Until about 10 years ago, virtually all children in American public schools were taught completely in English. The policy was painful for many newcomers but generally worked well in teaching the language and building a common American identity, said Stephan Thernstrom, a history professor at Harvard University who recently edited the "Harvard Encyclopedia of American Ethnic Groups."

Since the late 1960s, however, there have been strong pressures in another direction -- principally from Mexican-American, Puerto Rican, and other Hispanic groups. They argue that most of their children have fared poorly in English-lauguage schools and should be taught in Spanish at least as long as it takes them to learn English proficiently.

They have received some support from Congress in the Bilingual Education Act of 1968 and the U.S. Supreme Court in its 1974 Lau decision, though the court said simply that schools must do something special for the non-English-speaking students, but didn't spell out what should be done.

"From the viewpoint of [Hispanic] groups, they were simply submerged in [English] programs, and it was a very sad experience," said Rosario Gingras, a senior researcher at the Center for Applied Linguistics in Washington. "Their dropout rates are very high, their test scores low, and they felt why should they have to give up their language to satisfy somebody else's notions of what school should be like?"

But Harvard's Thernstrom suggested that the problems of many Hispanic children in U.S. schools may stem from impoverished homes and the low education levels of parents, rather than language difficulties. The children of some well-educated Hispanic groups, he said, notably the Cuban refugees of the early 1960s, have done well in American schools depsite having to learn English. Conversely, other children, notably low-income urbsan blacks and rural whites, have had serious educational problems even though English is their only language.

"Many children will have problems in school quite independent of language," Thernstrom said. "To think [bilingual] language programs are going to help them is probably mistaken because language isn't the problem."

So far, the evidence on whether bilingual education works is inconclusive. The laragest study, conducted by the American Institutes for Research in the mid-1970s, concluded that students in bilingual programs achieved no more than those with limited English in regular classes even without special ESL instruction.

Richard W. Tucker, director of the Center for Applied Linguistics, said the study has serious technical flaws and that a number of other studies "are beginning to appear that demonstrate the effectiveness of bilingual education." bBut Tucker added: "There are still just bits and pieces [of evidence] from here and there. There's no massive testing evidence that bilingual [special classes in English] is more effective."

"I supposr it gets down to . . . a question of value judgments," said Montgomery Blair principal Villani. "I just don't think [limited-English-speaking students] should have to pass regular courses if the work they have done is equivalent to an English program. We're not out to turn them into citizens of Kansas."