The relative decline of American education at the elementary- and high-school levels has long been a national embarrassment as well as a threat to the nation's future. Once upon a time, American students tested better than any other students in the world. Now, ranked against European schoolchildren, America does about as well as Lithuania, behind at least 10 other nations. Within the United States, the achievement gap between white students and poor and minority students stubbornly persists—and as the population of disadvantaged students grows, overall scores continue to sag.

For much of this time—roughly the last half century—professional educators believed that if they could only find the right pedagogy, the right method of instruction, all would be well. They tried New Math, open classrooms, Whole Language—but nothing seemed to achieve significant or lasting improvements.

Yet in recent years researchers have discovered something that may seem obvious, but for many reasons was overlooked or denied. What really makes a difference, what matters more than the class size or the textbook, the teaching method or the technology, or even the curriculum, is the quality of the teacher. Much of the ability to teach is innate—an ability to inspire young minds as well as control unruly classrooms that some people instinctively possess (and some people definitely do not). Teaching can be taught, to some degree, but not the way many graduate schools of education do it, with a lot of insipid or marginally relevant theorizing and pedagogy. In any case the research shows that within about five years, you can generally tell who is a good teacher and who is not.

It is also true and unfortunate that often the weakest teachers are relegated to teaching the neediest students, poor minority kids in inner-city schools. For these children, teachers can be make or break. "The research shows that kids who have two, three, four strong teachers in a row will eventually excel, no matter what their background, while kids who have even two weak teachers in a row will never recover," says Kati Haycock of the Education Trust and coauthor of the 2006 study "Teaching Inequality: How Poor and Minority Students Are Shortchanged on Teacher Quality."

Nothing, then, is more important than hiring good teachers and firing bad ones. But here is the rub. Although many teachers are caring and selfless, teaching in public schools has not always attracted the best and the brightest. There once was a time when teaching (along with nursing) was one of the few jobs not denied to women and minorities. But with social progress, many talented women and minorities chose other and more highly compensated fields. One recent review of the evidence by McKinsey & Co., the management consulting firm, showed that most schoolteachers are recruited from the bottom third of college-bound high-school students. (Finland takes the top 10 percent.)

At the same time, the teachers' unions have become more and more powerful. In most states, after two or three years, teachers are given lifetime tenure. It is almost impossible to fire them. In New York City in 2008, three out of 30,000 tenured teachers were dismissed for cause. The statistics are just as eye-popping in other cities. The percentage of teachers dismissed for poor performance in Chicago between 2005 and 2008 (the most recent figures available) was 0.1 percent. In Akron, Ohio, zero percent. In Toledo, 0.01 percent. In Denver, zero percent. In no other socially significant profession are the workers so insulated from accountability. The responsibility does not just fall on the unions. Many principals don't even try to weed out the poor performers (or they transfer them to other schools in what's been dubbed the "dance of the lemons"). Year after year, about 99 percent of all teachers in the United States are rated "satisfactory" by their school systems; firing a teacher invites a costly court battle with the local union.

Over time, inner-city schools, in particular, succumbed to a defeatist mindset. The problem is not the teachers, went the thinking—it's the parents (or absence of parents); it's society with all its distractions and pathologies; it's the kids themselves. Not much can be done, really, except to keep the assembly line moving through "social promotion," regardless of academic performance, and hope the students graduate (only about 60 percent of blacks and Hispanics finish high school). Or so went the conventional wisdom in school superintendents' offices from Newark to L.A. By 1992, "there was such a dramatic achievement gap in the United States, far larger than in other countries, between socioeconomic classes and races," says Kate Walsh, president of the National Council on Teacher Quality. "It was a scandal of monumental proportions, that there were two distinct school systems in the U.S., one for the middle class and one for the poor."

In the past two decades, some schools have sprung up that defy and refute what former president George W. Bush memorably called "the soft bigotry of low expectations." Generally operating outside of school bureaucracies as charter schools, programs like KIPP (Knowledge Is Power Program) have produced inner-city schools with high graduation rates (85 percent). KIPP schools don't cherry-pick—they take anyone who will sign a contract to play by the rules, which require some parental involvement. And they are not one-shot wonders. There are now 82 KIPP schools in 19 states and the District of Columbia, and, routinely, they far outperform the local public schools. KIPP schools are mercifully free of red tape and bureaucratic rules (their motto is "Work hard. Be nice," which about sums up the classroom requirements). KIPP schools require longer school days and a longer school year, but their greatest advantage is better teaching.