Who spends 15 hours a week studying in the library?

''Debate is the most elaborate ruse ever constructed.'' The speaker was William Branham, debate director at Bates, a small liberal arts college in this pretty area of Maine. He continued:

''Students motivate themselves to spend 10 to 15 hours each week in the library - thinking about issues, learning about communication, doing research - all without receiving really any tangible rewards.''

Branham should know. A top high school debater in Oklahoma and later at Dartmouth College during the early 1970s, he runs Bates's institute for high school students. It is one of about 50 similar institutes in the United States, often held in summer. The 1981 program drew on 74 high school students from 14 states to spend three weeks in intensive study.

The students, from 15 to 17 years old, are highly self-motivated, intelligent , and often very competitive. While many of their friends go out for sports and social activities, these teen-agers find a need for intellectual stimulation and satisfy it, at least partly, through the rigors of debate.

Debate institutes prepare students for high school tournaments that are held throughout the academic year. Institutes this year focused on the 1981-82 national high school debate topic: ''Resolved: that the federal government should establish minimum educational standards for elementary and secondary school in the United States.''

The institutes give high school debaters a leg up on the topic and research for the year. The Bates program also stresses fundamental theory and basic skills.

''The key to debate is that a position cannot only be asserted; it must be proved,'' Mr. Branham said.

In preparation, students bring into play many traditional disciplines that will help them in the future.

''Each student spends at least four hours in the library every day,'' Branham said. ''A good debater will gather an average of 10,000 bits of evidence over the course of a year.'' They keep all the evidence filed away in bulging suitcases that are carried to each debate.

In addition, debate also teaches other skills - organization, logic, persuasion, ethics, public speaking, analysis, ability to conceptualize a position, critical thinking, coping with pressure, and thinking on your feet.

Most of all, according to Branham, debate promotes originality. ''Schools often don't reward creativity,'' he said. ''In debate, however, creativity is all that's rewarded. Because it is competitive, students have an incentive to come up with a new approach.''

The pace at the Bates institute this year was rigorous. Panel forums were scheduled for three hours in the mornings; student groups met in the afternoon to work on cases, hold debates, participate in critiques, and do research. Meetings were also held in the evening, and the library was open in early morning and late evening for more research.

There were a few hours off each afternoon for a baseball game or swim, but then it was back to debate. The institute ended with a tournament and banquet.

The college's approach was decidedly low-key and informal, in contrast to some institutes which emphasize competition.

''We take the position that students will learn a lot more if they work as an entire group with a minimum of competitiveness, rather than trying to trick each other,'' said Branham.

That approach extended to the banquet, where the student who improved the most, rather than the individual who won the tournament, received the highest distinction.

''You have to take into consideration the amount of dedication the kids have, '' said Alfred Snyder, debate director at Wayne State University and one of the instructors at the Bates institute. ''They get one day off and work all other days - even evenings and weekends.''

Probably the best measure of success is the fact that a number of former institute participants return to Bates every year to serve as coaches during the tournament.

Alice Flaherty, a freshman at Harvard from New York, returned ''because of friends and close relationships with instructors.'' She may not continue with debate at college but realizes the value it had for her.

She pointed out that debate kept her aware of current events, taught her how to prepare speeches and term papers, and gave her finely tuned research skills and more.

''I like to argue at home - about current events, everything,'' she said, smiling. ''I just like to argue.''

Bates, an academically tough college, has been involved in debate since the turn of the century and was well known in international debate competition, then popular, from the 1920s through the '50s.

(It was the first US institution of higher learning to compete in international debate, against Oxford University in 1921.) During the 1980-81 academic year, the Bates varsity team was ranked among the top 10 in the nation, and one of its students, Stephen Dolley of Hermon, Maine, was named the top individual debater among college freshmen throughout the country.

Branham pointed out that high school students who debate show interest in all kinds of careers, from engineering to humanities, while college debate students usually head for careers in law or politics.

''Many national figures have come through debate,'' said Branham, ''Dick Cavett, George McGovern, and Edmund Muskie, an alumnus of Bates, to name just a few.''