America's students are returning to school, but in many cases, their teachers aren't.

The country is facing an education crisis as more teachers leave the profession for other fields, often lured by higher pay. At the same time, colleges have witnessed a plunge in the ranks of students majoring in education, leading to a shrinking pipeline of young classroom teachers.

States across the U.S. are reporting teacher shortages just as schools open for the 2018-19 school year. That's prompting a scramble among school districts: Earlier this month, Michigan held a job fair in a late bid to attract teachers to fill open spots, while a Vermont school official told the publication Vermont Digger "that almost every high school" was still searching for at least one math teacher.

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The teacher shortage emerged in the wake of the Great Recession, when school districts cut their staffing as funding dried up. But student enrollment has only grown, adding to the pressures on local schools. At the same time, fewer college students are opting to become teachers because of the economics of college debt, said Linda Darling-Hammond, the president and CEO of the Learning Policy Institute, a nonpartisan organization that focuses on education policy.

"There are studies about this that show people choose careers based on the salary in relation to the debt they have from college," Darling-Hammond said. "In many many states, salaries were frozen and never kept up with inflation."

She added: "People can't stay in a profession where they can't afford to support their own families."

The numbers of teachers leaving the profession for other fields has grown for each of the past three years, according to data from the U.S. Census Bureau. Most leave for jobs in health care or social assistance, which includes nursing, family assistance and child care. Some shift into administrative services like office work, Census said.

At the same time, fewer college students are studying education. Enrollments dropped by 35 percent between 2009 and 2014, according to the Learning Policy Institute.

Teachers are earning almost 2 percent less than they did in 1999 and 5 percent less than their 2009 pay, according to the Department of Education. Teacher pay hit the national headlines earlier this year when educators in states such as Kentucky and Oklahoma protested and went on strike because of low pay and pension changes.

Turnover is worse in districts and schools with low-income residents given the lack of resources to attract teachers, Darling-Hammond said.

It's especially hard to recruit in rural communities, she added. But those teachers are often burdened with many more stresses than teachers in wealthier school districts. "If you're in a tiny community, you'll earn less and have to pay more out of your own pocket for supplies," she noted.

Students from low-income districts may also be struggling with hunger, homelessness or trauma, she added. "Teachers are often the people who bear the brunt of that," she said.

Teacher shortages hurt students, but they also cost taxpayers: The Learning Policy Institute estimates the typical cost to find a replacement for an open teaching role is about $20,000.

Policy remedies could help end the teacher shortage, according to the institute. Those include mentoring programs and federal support for students who study education in college.

"We should say, 'If you teach, we will pay for your education,'" Darling-Hammond said. "We need a national policy that says teaching is a critical profession."