ST. LOUIS -- Like many high school students these days, Fred Rhodes works part time at a fast-food restaurant, enjoys music videos and reads few books for pleasure. But in the way he spends other hours of his spare time, he is something of a rarity.

Twice a week Rhodes, a senior at Sumner High School, comes straight home, cracks open his textbooks and does at least an hour of homework before going to his job. He regularly gets weekend assignments done on Friday evenings, a prime time for socializing for most teenagers. Altogether he devotes about 2 1/2 hours each school day outside class to his four academic courses at Sumner, where all 900 students are black and most are poor.

That amount of study time puts Rhodes among a minority of the high school students in St. Louis who comply with the toughest homework policy of any big-city district in the nation. A school board policy adopted in 1984 calls for high school students with a full load of six classes to average 3 1/2 hours of homework each school day. The 2 1/2 hours that Rhodes studies meets the policy's guidelines for a high school student with four courses.

But the reason Rhodes studies so hard has little to do with a homework policy that most Sumner students aren't even aware exists. His primary motivation comes from his parents, who value education and have instilled in their 17-year-old son a high priority on homework and a strong desire to attend college. "I know how important school is," said Fred Boyd, Rhodes's father.

For many educators and politicians around the country, promoting more homework seemed a common sense, inexpensive strategy for boosting student achievement, especially after a landmark 1983 report recommended "far more" studying for high school students. But persuading students to do it has proved more difficult. The homework policies adopted by many school boards have had little impact, according to experts, because schools simply cannot control how teenagers spend non-school hours or how much parents get involved in their children's studies.

"Even though it's a policy, it may not be followed," said Herbert J. Waldberg, a University of Illinois at Chicago professor who has studied student homework habits.

A 1984 article by Waldberg and two coauthors described homework as having "powerful effects on learning." School officals in St. Louis, like others elsewhere, say its value is the way it extends learning time, stimulates student thought and reinforces and applies classroom lessons.

A government-financed survey on homework released this year, part of the National Assessment of Educational Progress, found that 62 percent of 17-year-olds did less than an hour of homework each school day in 1988. About 26 percent did 1 to 2 hours, and 12 percent did more than 2 hours.

Most school districts with official policies on homework state that high school students should do 1 1/2 to 3 hours each school day, although teachers usually have the final decision over how much to assign and how much weight to give it in the final grade.

Few districts have tried to assess how effective the policies are or whether they improve student performance. St. Louis, for instance, has not done such a followup study. A district that has, Rochester, N.Y., found that half the 875 high school students surveyed in 1983 did not do the minimum recommended homework of 1 1/2 hours.

In the Washington area, most districts give teachers complete discretion in deciding how much homework to assign. In the few districts that have guidelines, the maximum or average daily homework for high school students is 3 hours in Baltimore County, 2 hours in Alexandria and 1 to 2 hours in Fairfax County. Montgomery County recommends homework three to five times a week and requires each school to adopt a homework plan in October.

A survey of students two years ago indicated that high school teachers did assign more homework in the late 1980s, but also showed that students were not doing much more. On average, students reported studying at most an hour each school day, or five hours a week, about the same amount as in 1980.

"There are some tiny little indications of more homework, but the truth is they don't do much homework," said Ina V. Mullis of the Educational Testing Service in Princeton, N.J.

In St. Louis, principals and teachers are supposed to develop a plan for coordinating assignments and ensuring that the homework policy is followed. One problem is estimating how much time an assignment will take, which often depends on a student's ability.

Teachers at Sumner appear generally aware of the tough homework policy, and routinely write homework assignments and lesson objectives on the blackboard before classes begin. But they sometimes let students do "homework" in class, a practice that does not extend learning time but often results in the assignment getting done.

"I would think if you get an hour {a night} out of them, you're doing good. Some of them you can't get to take a book home," said Shirley Cavitt, a Sumner counselor.

"Three hours, five days a week? Whoa. Normally, I have about an hour," said Nikey Shannon, 19, a senior. "If I get assigned more, I will do it."

Shannon, who occasionally helps out at the auto repair shop where his father works, said he studies the most for a black humanities course "because it's about my race." As for the rest of his homework, he said: "The other courses I finish up during the class time."

Kimara Henderson, a sophomore, said she does about 1 1/2 hours of homework two or three times a week. She spends about as much time at drill team practice as she does on homework.

"I don't like it or nothing. Sometimes it takes up my time. I'll do it. It's not a hobby," said Henderson, 16. "I don't really have much homework in a lot of my classes."

Christy Balentine, a 17-year-old said she also studies 1 1/2 hours a night, about half as much as she should under the homework policy. "Three hours is too much homework," said Balentine, who runs track and wants a part-time job to pay for clothes and save for college.

Not surprisingly, Japanese students who top American ones in comparative international testing also outdo them in homework. A 1985 study found Japanese high school students did an average of 19 hours of homework a week, about four times as much as their U.S. counterparts. By doing an average of eight hours a week, even Japanese elementary school pupils were studying harder than most U.S. high school students.

American students who do the most homework tend to be in college preparatory programs, as Rhodes is. Asked why he studies so hard, Rhodes replied: "I know I want to go to college."

While some college-bound students do minimal amounts of homework, the 50 percent who do not go to college have the least incentive to do any more than the minimum required to earn a diploma. Lately, their lack of motivation to study is getting attention from educators and business executives dissatisfied with the quality of entry-level workers.

"It's the students who expect to go into employment after {high} school who you are going to have to motivate," said Christopher T. Cross, assistant secretary of education for research and improvement. He said those students "haven't seen any economic consequences for doing it {homework}."

The American Business Conference, which represents 100 medium-sized firms, plans a national project to get businesses to consider school transcripts and test scores in deciding whether to hire recent graduates and how much to pay them. Pilot projects are to begin next year in Orange County, Calif., Fort Worth, Tex., and Morris County, N.J.

Joseph DuBose, Sumner's principal, said he tries to motivate job-bound students by warning them that employers will check their personal references at the school. Local businesses do inquire, he said, but more frequently about students seeking summer work than about graduates looking for permanent jobs.

Educators say it is impossible to overemphasize the role of parents in motivating their children to do homework. But efforts to involve parents in ensuring homework gets done have been spotty.

Cavitt, Rhodes's counselor, credits his parents for his diligence. Both are postal workers and attend parent-teacher meetings regularly. "It's his home life. They want him to achieve," Cavitt said.

Boyd said he emphasized to his son that getting a job was going to require more school than he had had. "I told him nowadays -- we were lucky to come right out of school and go to {work at} the post office -- you can't just have a high school education and have a good job," he said.

Boyd said the strict rules on homework imposed on his son when he was younger developed self-discipline. Those rules, he said, were "do your homework first, before anything else" after school and "get it out of the way Friday evening, then he can have his weekend."

When he was a junior, Rhodes's grades dipped after he took a part-time job frying chicken at a fast-food restaurant. Cavitt said a girlfriend contributed to the academic letdown. "He was in love and he was working late, too," she said.

The slide halted after his father called him on it. "I told him he had to quit his job, and that's what made him buckle down because he really wanted that job," Boyd said.

Now Rhodes is a manager at the restaurant, which allows him to bring a textbook along when he begins work at 5 p.m. two school nights a week.

At school, Rhodes often studies through his half-hour lunch period. One recent day, he sat on a wooden bench in the front hallway, eyeing a notebook spread on his lap, pulling potato chips from a jacket pocket and sipping a soda. As usual, he was studying Western civilization, his most difficult course, which meets right after lunch.

"It's not that it's hard," he said. "It makes me work."

Staff researcher Ralph Gaillard Jr. contributed to this report.