After 24 years in business he took up teaching - and won't quit

''I hope I never have to retire,'' says the indefatigable instructor, who chose teaching as a second career when he was 51. In an era when teacher burnout and lower than average pay scales force many professionals out of the classroom into industry or early retirement, Marten Tafel, in his mid-70s, is a notable exception. Last month the Westport, Conn., teacher won the Distinguished Service to Science Award, presented annually by the National Science Teachers' Association.

''To say 'Marty' has become a legend in his own time is an understatement. His dedication and enthusiasm . . . motivate those who are around him,'' says the state's science consultant, Dr. Sigmund Abeles, who initiated the nomination.

''Teaching is the one career that has given me inner satisfaction,'' Mr. Tafel affirms. He worked in New York's Garment District for 24 years before he decided to change jobs in mid-life. He returned to college in 1959, earned a master of science degree from Yeshiva University in New York City, and was hired as an intern at Long Lots Junior High School in Westport, teaching physics, biology, and science to middle-school students.

The assignment marked the beginning of a rewarding career. But in 1980, with 21 years in the classroom as a tenured teacher, the State Teachers' Retirement Board told Tafel that his career had ended because he had reached the mandatory retirement age of 70.

With the support of the Westport Board of Education, the teacher won the right to return to the classroom when the Connecticut General Assembly repealed the regulation. The decision was a victory for Tafel and for Connecticut's 40, 000 public school teachers, who may now elect to remain in the classroom beyond 70, provided their annual contracts are renewed by their local board of education.

Last September, when school enrollments in Westport declined and grades were realigned, Tafel was assigned to teach beginning biology at Staples High School. Taking the change in stride, he is up before dawn when school is in session. He reports to work by 6:45 a.m. to review plans for classroom work and lab experiments. He designs the experiments to test the knowledge of his 100 9th-, 10th-, and 11th-grade biology students.

In the classroom his manner is stern, yet mixed with a quick wit and intense curiosity. ''I can only open the door; the real learning is up to them,'' he says after a discussion in which teacher and students have volleyed questions about photosynthesis. ''The process requires energy to create, a little less energy to take apart. It's like lighting a firecracker with a small match,'' he explains.

Marten Tafel has no patience with critics who claim young people today are undisciplined and lack respect for adults, their community, and their nation. ''Kids haven't changed; it's the whole society that is changing around them,'' he says. ''We are the ones who must set the example; then we must let them choose what is right and what is wrong.

''They learn at a faster rate than we learned 20 or 30 years ago, as the technology of science explodes, but they can handle change,'' he affirms.

In an article in the National Science Teachers' Association Journal in 1981, Tafel focused attention on the need to help students allay fears of a potential nuclear holocaust. He presented the pros and cons of nuclear fusion, emphasizing the possibility of progress, not destruction.

For 21 years he has been active in the Connecticut Junior Science and Humanities Symposium, bringing together leaders in industry and education to share innovative projects and ideas with gifted students. He has encouraged scientists to come into his classroom for a day.

Tafel advises graduates who earn a degree in education to do their fieldwork in business or industry before they enter the teaching profession. ''Unfortunately, today the reverse is true, which may explain the high incidence of teacher burnout and early retirement,'' he says.

''I would resign if I knew I was not reaching the students or if I knew I was keeping a younger, more qualified teacher out of the classroom,'' he remarks. ''I am optimistic that I will be back in the classroom next year, marking a quarter-century in public school teaching.''

''One of the things that has distinguished Marty Tafel as he became more prominent in science education is the fact that he did not move out of the classroom,'' says James Rutherford, chief education officer at the American Association for the Advancement of Science.

''Quite the contrary,'' Mr. Rutherford continues. ''He has fought to stay there, knowing that his most important contribution is to the students. His writings and his participation in (the Connecticut Science Teachers' Association) and NSTA and professional associations is energized by his continuing classroom teaching. This has been Marten Tafel's strength.''