What prompts so many young teachers to get out of the profession?

American schools will need roughly 1 million new teachers by 1990, say experts. Yet of the current crop of teacher recruits, about half -- usually the best and the brightest -- will quit by their fifth year, according to the National Education Association. Why is that? Ask Karla Hooper.

Six years ago, Ms. Hooper started teaching biology at Marin Academy -- a small, private high school in San Rafael, Calif., an upper-middle class Bay Area suburb.

For the first three years, Ms. Hooper says her job was all she had hoped it would be. But then, much like an aircraft approaching the sound barrier, she hit what felt like an invisible wall. Teaching lacked its original sparkle. And not even the beautiful setting, motivated students, and sympathetic faculty warded off thoughts of leaving the classroom.

``About the fourth year, I lost the enthusiasm based only on idealism,'' she says.

In a phenomenon distinct from classic ``teacher burn-out'' from job pressures and overwork, young teachers today -- weaned in a society offering much more choice, especially for women and minorities -- often find in their first few years a tough set of circumstances, questions, and decisions.

Not all make it through ``the invisible wall,'' as did Hooper.

David Mallery, director of professional development for the National Association of Independent Schools, runs an annual teacher's workshop titled ``The First Five Years.'' Twenty years ago, he says, new teachers used to quit early to join the Peace Corps. Today they quit to study law. It is roughly in their third year, he says, that teachers ``become vulnerable to law school'' -- primarily for the money.

``It became more apparent,'' says Hooper of her struggle, ``that I had to ask: How does society perceive me?'' Teachers ``aren't acknowledged for the importance of what they do. Society tends to think the most successful people are the ones who make the most money. Yet teachers are critical. Today, many of the children I see are more influenced by teachers than by their parents. Teachers become the role models.''

Besides having to deal with the standard difficulties (long hours, undisciplined students, abrasive parents or administrators), young teachers fall into a variety of other traps, says Mr. Mallery. One of these is simply the fact of being new: You look younger than the other teachers, he says, and kids find that exciting.

``It's kind of heady, sometimes, to suddenly be the new teacher on the block,'' Mallery says. ``You are neat and young, and the kids hang onto you.''

The problem comes about three years later when a teacher's newness wears off and he doesn't find himself the center of attention so often, says Mallery.

A related problem, he says, is a condition of deflated expectations, when new teachers see they are not having the impact they assumed they would. After three years, some teachers start to say, ``Hey, I came here to help students learn about history and life, and I'm not doing it.''

Not all starting teachers have trouble keeping the fire burning. Heidi Tamas, an elementary teacher for five years in Menlo Park, Calif., says she has never lost her enthusiasm for teaching.

``I love it,'' she says. However, she constantly works to overcome stagnation. She's getting a master's degree in education ``just to stay sharp,'' she says. ``After talking with fourth-graders all day, you sometimes forget how to communicate with adults.'' She also ``team teaches,'' and says her principal is extremely supportive. ``I had a horrible class last year,'' she says. ``The principal knew it, and within a day was on top of every request I made. That makes a difference.''

Such perks as merit pay, specialized field trips, and private workshops are one way private schools such as Marin Academy keep young teachers interested. But Marin headmaster Bruce Shaw tries to keep his teachers ``fresh'' through some low-cost methods, too. Since lack of feedback is a notable problem in teaching, Mr. Shaw has set up after-school meetings to discuss teaching styles, student behavior, and ways to help students think critically.

Susan Rosenholtz has been researching the teaching profession for several years at the University of Illinois: ``The most striking thing about the research is that when we look at the most effective schools,'' she says, ``there is an altogether different pattern of faculty interaction. It is an extremely collaborative setting.'' The simple act of having teachers talk with one another makes a big difference in their attitude and performance, she says.

Rosenholtz reports that new teachers, faced with the ``reality shock'' of the classroom and its behavior problems, tend to forget their college teaching models, and learn primarily by trial and error -- often falling back ``on memories of teachers from their own student days.'' The trick, she says, is in breaking that cycle.

What finally helped Karla Hooper renew her desire to teach was a process of ``backing up'' from her original notions of the classroom, rethinking why she wants what Marilyn Rauth of the American Federation of Teachers describes as ``a job that is much more complex than young teachers imagined.''

People may never know what teachers have done or what they've given up, Hooper says, so teachers must find their satisfaction ``from within.'' They have to come to know the value of what they are doing, even if others may not. ``It's important I do something I believe in,'' she says. ``That outweighs the money factor.''