About one in five teachers leaves the profession within three years of entering the classroom, and those who leave are likelier to have entered college with higher test scores than those who keep teaching, according to a new analysis of federal survey data.

That nationwide pattern in teacher turnover suggests that "the best and brightest" abandon the classroom much faster, the trade publication Education Week concluded in its annual report on education, "Who Should Teach?"

Among teachers who graduated from college in 1993, those who had left the field by 1997 were about twice as likely to have scored in the top 25 percent nationally on the Scholastic Assessment Test or American College Testing program than those who were still teaching, the study found in its review of Education Department surveys.

Working conditions, student misbehavior and relatively low salaries were the most common reasons that teachers left.

"The best and brightest are most likely to leave. They are the ones who feel they have the least control over the school environment," said Virginia B. Edwards, editor of Education Week and the new report.

Such departures damage a national effort to upgrade teacher quality--shown in other studies to be an important factor in student achievement.

Despite the shortages, the gap between teacher salaries and those of other professionals has been growing, the study found.

From 1994 to 1998, the salaries of teachers with master's degrees increased by less than $200 a year, after adjusting for inflation, compared to $17,505 for other workers with the same level of education. About half of teachers have master's degrees.

Even though unionized teachers generally get raises based on seniority, the study found that the salary gap also widens with senority. Teachers in their twenties earn almost $8,000 a year less than other graduates the same age, but teachers in their late forties get paid about $24,000 less than their college-educated age mates.

"It's shocking that the gap is so great," said Bob Chase, president of the National Education Association. "For all the discussions about schools adopting efficient business practices, why should people believe the laws of supply and demand end at the school house door?"

Schools are less responsive to changes in the job market because most are under public control, Edwards suggested.

School systems also evade market pressure to raise teacher pay, she said, by waiving their own job requirements and hiring underqualified teachers. The study found that while 39 states require prospective teachers to pass a basic skills test, 36 permit hiring applicants who have flunked.