There was a time when Zina Greene loved teaching so much, she said, that "when the bell rang at the end of the day I hated it."

But after 15 years as a science teacher in Montgomery County, Greene quit her job in December to work for an executive placement firm in the District, where she can earn twice as much as her yearly $36,000 teacher's salary.

Yet Greene said money was not the only factor in her decision to leave. She said poor working conditions finally drove her to change professions.

During her last year of teaching, she said, she did not have her own classroom and had to share office space with four teachers, leaving her with "no place to go to be by myself" to prepare lesson plans or talk with her students.

"I didn't feel like a human being anymore," she said.

Greene is not an isolated example of discontent among Montgomery County teachers.

In a recent poll of 4,684 school staff members done by the Montgomery County Education Association, the union that represents 6,400 teachers and staff members in the county, 40 percent of the teachers who responded to the survey said they were "seriously considering" leaving teaching.

"I was shocked," said Mark Simon, president of the association. "I had expected that in Montgomery County it would be a lot lower."

In a similar survey of Prince George's County teachers done last year, 50 percent of the teachers who answered the survey said they were "seriously considering" leaving the profession.

A Harris survey done last year of teachers nationwide found that one in four believes he or she is likely to leave the teaching profession in the next five years.

Among their reasons for wanting to leave, Montgomery teachers cited large class size, problems with student discipline, inadequate salaries and insufficient time to fulfill their job requirements.

About one-quarter of those who said they were seriously considering leaving said it was because they were at retirement age, even though retirement is not mandatory for teachers in Maryland.

The threat of an exodus of teachers comes at an inopportune time for Montgomery schools, which like other local systems are facing the prospect of a teacher shortage. That shortage is growing around the country because many teachers are reaching retirement age and the school age population is booming.

Montgomery school officials estimate that they will have to hire 4,000 new teachers, more than half the work force, during the next five years, a number unparalleled since the late 1950s and early 1960s when the postwar baby boomers began pouring into the nation's public schools.

About one-third of those new teachers will be needed to replace teachers who retire. The remainder will be teachers needed to teach the children of baby boomers who are beginning to swell elementary school classrooms.

Researchers project a need for 1 million new teachers nationwide by 1990. Science, mathematics, language and special education teachers already are in short supply.

And the number of students going into teacher colleges across the country has declined in recent years, according to school officials.

Anticipating how hard it may be to find new teachers next year, the Montgomery County Board of Education recently offered to raise beginning teacher salaries 16 percent next year, from about $17,000 to $20,000, in an effort to be competitive with surrounding school systems. The average teacher salary in the county is $31,268.

The teachers union rejected the offer, saying it would benefit only teachers at the lower end of the salary scale.

The looming teacher shortage, the need to hire so many new teachers, and dissatisfaction over salary and working conditions are worrisome to school officials.

"All those trends are going against each other to create a situation that will be very difficult," said Michael O'Keefe, president of the Commission on Excellence in Teaching, a special commission created by the Montgomery County Board of Education to study the teaching problem.

"It's a very sizable challenge for a system like Montgomery County."

While school officials admit they are concerned about the local poll results, they say they are not overly alarmed that there will be a mass exodus of experienced teachers.

They said Montgomery County teachers generally stay through retirement age and note that the attrition rate for teachers is low -- about 7 percent left during the last school year.

"I was puzzled in terms of the number of people who said in the survey they were considering leaving against actual practice," said Superintendent Wilmer S. Cody.

Last year, for example, 440 teachers left the school system. Of that number, 116 retired.

But Simon said many school officials are not taking the problem of low teacher morale seriously enough.

"There is a lack of recognition on the part of school administration to this problem," he said. "For the system to take that attitude is scary."

One of the teachers who opted to leave is Debbie Johnson. She quit her job as a special education teacher last year after six years in the Montgomery system. Although she was making $24,000 a year when she left, she took a $10,000-a-year job as a sales clerk at Conran's in north Bethesda.

"I needed a year to cool out," she explained.

Johnson, 29, said she became disillusioned with teaching when she began to "feel a dichotomy between how I was taught to teach learning-disabled kids and what could feasibly be done in the public school system."

Although Johnson spent several hours every day, outside the regular school day, preparing lesson plans, making special learning aids for her pupils and correcting papers, she said she still did not have enough time to devote to each of her students.

The frustration and stress began to wear her down, she said, causing her anxiety and back pain. "It would take me hours after I got home to get it out of my system," she said. "I complained a lot to my husband."

But now, after several months as a sales clerk, Johnson said she is getting bored. She said she will consider returning to teaching when she moves with her husband to Las Vegas this year.

Sandy Hoyt, a teacher for 17 years, quit her job at Rock Creek Forest Elementary School last year after the combined fourth- and fifth-grade social studies class she had been teaching with another teacher was split up.

Now she stays home, spends more time with her children and tutors two students several times a week.

She said the split-up of her class, on which she was not consulted, was just one aspect of her "mounting dissatisfaction" with the school system. Hoyt said she got tired of fighting the school bureaucracy, having to teach from an inflexible curriculum, and worrying about how well her students were doing on achievement tests.

Hoyt said she believes that the county school system has little respect for teachers.

"It's in our contract that we don't have to call our own substitutes, but when you wake up in the morning with the flu, if you want the substitute to be there when school starts you have to spend an hour to an hour and a half trying to reach a substitute," she said. "That's very humiliating."

Greene, 44, who gave up teaching for the executive placement business, said she finds her new profession challenging, and there is a noticeable difference in her mood.

"She isn't as tense," said Greene's husband Richard, an accountant.

"I don't feel like a teacher or think like a teacher anymore," said Greene, who has two children, ages 16 and 10. "I come home from work now and have emotional energy left over for my family."

But teaching is still a powerful attraction.

"I think it's a real pity I'm not teaching anymore," Greene said wistfully. "I'd go back if I knew I was going to teach subjects I liked, if I wasn't treated as a rookie and if I had my own classroom so students would know where they could find me."