"My image of America was of bright lights, the typical downtown idea, you know, something glitzy," said Koya, who was recruited in 2000 to teach science on Chicago's South Side. "But you soon see that students are pretty much the same everywhere."

Koya, who holds a master's degree in zoology, is one of a small but growing number of foreign-born teachers in U.S. public schools, and may be on the vanguard of a broader trend.

As baby boomers — people born between 1946 and 1964 — start to retire in large numbers, experts say highly educated immigrants like her could become much more important to the U.S. labor market.

"All these baby boomers are going to be leaving these jobs at once, and increasingly we're going to rely on foreign-born workers to fill them," said Paul Harrington, an economist with Northeastern University's Center for Labor Market Studies.

The millions of immigrant workers will be crucial in another way: If they provide enough of a boost to the taxable wage base, it could help extend the viability of sorely stretched social programs that retired boomers will be counting on in their old age.

If nothing changes, current government projections indicate that by 2019, Medicare will be broke and the Social Security system will begin paying out more in benefits than it collects in taxes. The Social Security Trust Fund will be exhausted by 2042.

The crisis is being driven in large part by boomer demographics. By 2050, the census projects, there will be 87 million people over age 65, compared with 35 million in 2000. Of those, 20 million will be over the age of 85, compared with just 4 million in 2000.

But immigration isn't a panacea.

While new arrivals might help expand the wage base, that alone isn't enough to save Social Security and Medicare, said Laurence Kotlikoff, a Boston University economist and co-author of "The Coming Generational Storm," an examination of the fiscal problems America will face as boomers retire.

"These very-old old people are going to be very expensive to take care of," Kotlikoff said. "I could see a collapse of our fiscal system just because the demands for healthcare benefits are going to be so high."

And while foreign-born workers will help replace baby boomers, it's not clear how much more immigration can rise. In terms of absolute numbers, immigration is currently at record highs.

Foreign-born workers accounted for almost half of U.S. labor market growth in the 1990s, up sharply from just 10 percent in the 1970s and 25 percent in the 1980s, according to the U.S. Census Bureau. The largest numbers come from Mexico, the Philippines, India and China.

Immigrant workers do pump up the tax base, but not as much as you'd think, Kotlikoff said. Unless they're very highly skilled, they can't generate enough taxes to outweigh the costs of the services they use, such as welfare, emergency medical care and public education.

To counter this, economists suggest U.S. immigration policy will need to be adjusted in the coming decades. The policy is now focused largely on family reunification — meaning people are more likely to be granted residency status if they have relatives here — instead of being decided by skill level, which many other developed countries take into account.

The teaching profession is a case in point. With many boomer teachers nearing the end of their careers, it may become increasingly difficult for the nation's public schools to fill a projected 2 million vacancies over the next decade.

In addition to stepping up domestic recruitment efforts, urban districts such as Chicago have sought foreign-born workers for hard-to-fill math, science and language posts.