''Both feet planted firmly in the air.'' This is a common response to Theodore Sizer and his recently published study of secondary education in the United States - ''Horace's Compromise: The Dilemma of the American High School'' (Houghton Mifflin Company).
It comes from critics who consider Dr. Sizer's recommendations for school reform hopelessly impractical or, at the least, applicable only to elite prep schools. It also comes from supporters who insist his idealistic standards are exactly what is needed to restore quality to America's schools.
Before the five years of research that led to his new book, Dr. Sizer was involved in education at the graduate and at the secondary levels. He is former dean of the Harvard Graduate School of Education and former headmaster of one of the country's leading preparatory schools, Phillips Academy at Andover, Mass.
In an interview in a New York hotel suite, where he appeared as relaxed as if perched behind the desk in the study of his home in Harvard, Mass., Dr. Sizer explained some of his conclusions about high schools, conclusions which depart radically from the findings in a flurry of reports on education published this past year.
Dr. Sizer believes, for example, that, if secondary education is to be truly effective, high schools should be open only to students who've demonstrated basic competence in literacy and mathematics and who possess an understanding of civic responsibility.
He argues that a system of Socratic questioning, rather than uniform course structures, is what is needed to improve learning in high schools.
He would restructure the academic program into four large departments: inquiry and artistic expression, mathematics and science, literature and the arts, and philosophy and history. The curriculum is overloaded with courses, he says. Schools unwisely value ''mere'' coverage more than mastery of intellectual skills. Too often schools are judged on the basis of data that happen to be easy to collect. The statistics can be manipulated without giving a true idea of what is learned. Just because time is spent on a subject, that does not mean the subject is learned, he says.
He would change the practice of grouping students by age into grades. Students learn at different rates, he says. Just because a student is 15 does not automatically mean he or she is two years away from graduation. Time is but one factor in learning. Motivation and learning style are more important, he argues.
The Horace to which the book's title refers is ''Horace Smith,'' a hypothetical 53-year-old English teacher, a composite of teachers Sizer has observed and interviewed. Smith's compromise comes from his daily actions and deliberations in trying to strike a balance between what students need to learn and the time and energy he has available in any given day.
''Horace Smith is part of the 'deal' students and teachers have made in classroom after classroom across the country not to hassle each other by having too high an expectation of what schooling should be,'' Sizer says. Because of that deal, school becomes an intellectually calm and undemanding place.
Sizer charts a 180-degree turn in the understanding of high school - from an institution in which adults deliver services to adolescents to one that coaches youngsters in the development of mind and character - and he feels this understanding must be student-driven. ''The key workers must be the students. Only they can do the learning,'' he says.
Any from-the-top-down approach to teaching is anathema to Sizer. It just won't work, he says. What is needed, instead, is grass-roots flexibility, a system in which faculty and students control curriculum and class structure. The focus of any effective school must be the ''critical triangle of the student, the teacher, and the thing to be taught,'' he says.
When challenged with the argument that his new structure is elitist and impractical, Sizer is adamant that students must leave school with self-confidence in their ''effective ability to teach themselves. What I am proposing is what I would want my own children to have, and so does every other parent who is a smart consumer of education. They want their kids known, and they want their kids known by people in whom they have confidence and respect.''
The group that some people ''think I am pushing out of the schools is the very group which I am emphatically saying the state must hang with, well beyond the age of 16 or 18 in many cases,'' he adds. ''If you look at who is the demoralized youngster, who is cutting school, who is truant, and about whom we worry the most, it's that student who is most likely illiterate, who can't have an effect on his or her life.''
Sizer was most struck by the ''sameness'' of the schools he studied, and the ''acceptance of mediocrity.'' Organization of American high schools arises from ''the beliefs of the 1890s,'' he notes. ''We have learned much and changed much since then, and we can do better than continuing to operate a school designed when Henry Ford's Model T was new.''
California Superintendent of Public Instruction Bill Honig, although not willing to go as far as Sizer with some of the radical reforms the former Harvard dean suggests, does agree with his central theme. ''You don't engage kids with multiple choice and workbook exercises in high school,'' Dr. Honig says. ''The need is to get in deeper to essays and classroom discussions.''
In the study which resulted in Sizer's book and which is continuing even after its publication, the former English, mathematics, and social studies teacher has the assistance of some two dozen researchers working in 80 high schools. The book, begun in 1979, was sponsored by the National Association of Secondary School Principals and the National Association of Independent Schools.
Albert Shanker, president of the American Federation of Teachers, praises Sizer for putting the emphasis on the encounter between teacher and student and for writing the report that best speaks to actual conditions in the classroom. Sizer explicitly calls for a daily 80-to-1 student-teacher ratio.
From his new post as chairman of the Education Department at Brown University (effective in July), Sizer will lead a Coalition of Essential Schools, each of which has agreed to adopt principles outlined in ''Horace's Compromise.'' From five to 15 school districts that are geographically diverse, public and private, urban and rural, will take part in a 10-year demonstration project.
No school is obliged to follow such particulars as eliminating age-based grade levels or vocational courses, but it is safe to assume that a streamlined curriculum would be a central commitment for selection in theproject. One critical test of practicality, to which Sizer hopes to hold each district, is an agreement to carry out any changes without raising per-pupil expenditures by more than 10 percent.