The teaching profession is transforming, according to research released Tuesday: It is larger than ever, but more unstable; it's attracting more educators of color than ever, but losing them at higher rates than white teachers.

The Great Recession brought school layoffs, but that trend has reversed, and the number of teachers nationwide is increasing far faster than the rate of students, according to federal data. Between 1988 and 2016, the number of teachers nationwide increased by more than three times the number of students.

"The growth in teaching has been spectacular, and our point is, there is going to be a price," said Richard Ingersoll, a professor at the University of Pennsylvania's Graduate School of Education, who has been studying the profession for 30 years. Ingersoll is lead author of the updated research. "I don't see this growth as sustainable."

There have been steep increases in special education teachers, educators who instruct English language learners, and elementary-level enrichment teachers — those who teach a subject like foreign language or robotics. There's been a 90 percent increase in math teachers, and a 94 percent increase in science teachers — due in part to changing graduation requirements in schools across the country.

"Some critics say this increase of teachers is an example of inefficiency, but in all of these fields that have increased, it's in response to demand," said Ingersoll. "The public wants ESL and bilingual teachers. They want math, and science, and foreign language teachers."

Slower student growth and more teachers does not necessarily equal lower teacher productivity, the researchers found: Many of the areas of teacher growth, such as special education, require intense workloads and small class sizes.

But the hiring comes at a cost. Over two decades, the additional cost of teacher salaries above what would have been necessary to match student enrollment shifts was over $30 billion, the researchers estimated.

And while the number of teachers is soaring, schools are struggling to retain teachers; 44 percent of teachers leave the profession within the first five years. The reasons teachers cite are varied, but most often deal with "dissatisfaction with any of a variety of school and working conditions, including salaries, classroom resources, student misbehavior, accountability, opportunities for development, input into decision making, and school leadership," according to the research.

That churn is often concentrated in districts like Philadelphia: poor, urban school systems that educate mostly children of color. These schools have the highest rates of turnover, and nearly half of all public school educator turnover happens in a quarter of public schools.

The teaching force is aging, with pension and school budget implications, but because of the increase in teaching positions, there's also a concentration of new teachers, many of them recent college graduates.

Education, long a female-dominated field, has grown even more so: Both the number of women entering the field and the percentage of female teachers has increased. The number of males who enter teaching has also increased, but the number of females has increased at twice that rate.

"If the trend continues, we may see a day when eight of 10 teachers in the nation will be female," the researchers wrote. "An increasing percentage of elementary schools will have no male teachers. An increasing number of students may encounter few, if any, male teachers during their time in either elementary or secondary school."

The teaching profession has attracted more women but also more minority teachers, which may seem counterintuitive, given many districts' struggle to attract teachers of color. The data show, however, that the number of minority teachers is actually growing faster than the number of minority students, an "unheralded victory," and likely indicates that a proliferation of recruitment programs for teachers of color has benefited the profession, Ingersoll said.

Still, there's a significant mismatch in many districts — such as Philadelphia's — between the percentage of minority students and that of minority teachers. That gap remains in part because the quit rate for minority teachers is much higher than that of white teachers, and is growing.

The findings raise many long-term questions for America's schools, Ingersoll said, and more research is needed.

"At this point, we have more questions than answers," the report said. "What is clear is that large-scale changes are happening to the nation's largest occupational group."