Common sense critique of schools, Free to Teach: Achieving Equity and Excellence in Schools, by Joe Nathan. New York: The Pilgrim Press. 224 pp. $14. 95.

This sort of book often doesn't get written. It's a midstream critique of today's schools by a front-lines educator - the kind of practical thinking usually found in memoirs at the end of a career.

''Free to Teach'' comes from an unlikely but most appropriate character - an assistant principal (nobody's favorite job, he says) at a junior-senior high school in St. Paul, Minn. Author Nathan's perspective and experience lend fresh insight to the current debate about the quality of US education.

Nathan - who has been both teacher and administrator in both a ''traditional'' and an ''open'' school - insists schools can become excellent without spending more money on them. He's an inspiring advocate of projects that involve youth in the community, of graduation requirements that include career and consumer awareness and include practical use of such ''basic skills'' as reading, writing, and math, and of computers - one for every student. He says certain reforms are possible given the current school setup, but he advocates a more radical reform - a voucher system in which tax money goes directly to families, who decide how to spend it on education.

Nathan is at his best when he describes his own experience. His opening chapter on a day in the life of an assistant principal should be required reading for everyone interested in improving education. His empathy with children and his constant referring to the rights and responsibilities of citizens outlined in the US Constitution as guidance for solving tough problems show how an administrator can stay sane and humane.

He talks openly about his failures and problems. He describes the importance of complimenting an eighth-grade girl on her new haircut. (''She beamed. People like to be noticed!'') And he talks about discipline. On a trip to Chicago, he writes, ''A few students decided to stay up late on the first night and made more noise than was necessary, so I got them up at 5 a.m. and took them out for morning exercises in the park. That stopped the late night noise.''

Joe Nathan will win no literary prizes for this book. The book rambles occasionally, and lacks transitions. It preaches, usually with enough facts and support to make the preaching unoffensive. Parts of the book are repetitive, as if a series of memos and working papers were assembled without really making them into a book. (In the middle of the chapter on discipline, there's a long but related discussion of teaching styles.)

Still, the book's content is its important contribution to the ongoing educational crisis. Its stress on high expectations of performance by teachers and students, its talk about the recent innovations that have proved successful - including the idea of schools as community service centers and ''school site management'' programs - involve staff, parents, and students in making decisions about how a school operates.

Out of Nathan's day-to-day experience, ideals, personal vision, and unabashed frustration emerges some clarity that could prove useful to policymakers, to school staff, to parents, even to students.

Maybe, like Joe Nathan, parents and educators who read this book will know how to do something to help direct the tremendous human energy that surges out of the school building when the final bell rings.