President Barack Obama had more homework than usual this week. In the wee hours of Monday morning, Obama surprised administrators at Washington’s Sidwell Friends Academy, where his two daughters attend school. He and his wife showed up for the quarterly ritual known as the parent-teacher conference. After hearing about Malia and Sasha from their teachers, Obama stopped by Viers Mill Elementary School in Maryland, where he led a group of local students in chants of “read, read, read, read!”

The whole first family has been focused on education. The first lady penned an op-ed in U.S. News and World Report in which she sang the praises of the men and women who are training the next generation of America’s leaders: “We all remember the impact a special teacher had on us—a teacher who refused to let us fall through the cracks; who pushed us and believed in us when we doubted ourselves; who sparked in us a lifelong curiosity and passion for learning,” she wrote, citing data that shows “the single most important factor affecting students’ achievement is the caliber of their teachers.”

Michelle is right: A 2006 Brookings Institution report notes that “having a top-quartile teacher rather than a bottom-quartile teacher four years in a row would be enough to close the black-white test score gap.” In the face of distressing recent reports that the racial achievement gap is as wide as it has ever been, that is an important statistic.

But is any teacher a good teacher? Is a smaller class a better class? That’s been the subject of a fierce debate in education-policy circles for years.

Lawyer and media mogul Steven Brill jumped into the fray with a lengthy screed about bad teachers last month in New Yorker magazine. He argues that teachers’ unions are too powerful; that the political cost of firing bad teachers is so great that the city of New York tolerates enormous monetary costs just to avoid doing it. One such teacher, “Patricia Adams,” was found passed out in her classroom: ‘There were 34 students present in [Adams’s] classroom,’” Brill reports. “When the principal ‘attempted to awaken [Adams], he was unable to.’ When a teacher ‘stood next to [Adams], he detected a smell of alcohol emanating from her.’ ”

Needless to say, this doesn’t describe the teachers at posh Sidwell Friends. And the Adams case is an extreme example of incompetence. But what is to be done about the bad apples teaching the 50 million school-aged children, including the majority of black children, in the nation’s public school system?

Obama’s first instinct has been to protect teachers from being fired—not as ideology, but as economic stimulus. State budgets slashed as a result of the ongoing recession put thousands of firefighters, police and educators on the chopping block. The president and new Education Secretary Arne Duncan have tried to stop the bleeding. In a new report released by the Domestic Policy Council, some 250,000 teaching positions have been created or saved as a result of the American Reinvestment and Recovery Act. Why is this good? “Fewer teacher jobs, without fewer students, may lead to larger and often unacceptable class sizes,” read the study.

But merely keeping a warm body in front of children is not enough to “prepare them for a 21st century economy,” as Obama has urged. Bad teachers, according to a recent study by McKinsey and Company, send their students into what’s essentially a permanent economic depression—with fewer career opportunities and less earning potential over the course of the working lives. And when the New Teacher Project surveyed veteran educators from across the country, asking them to name the proportion of their colleagues whom they thought provided poor instruction, the teachers said 3-8 percent. “That’s lots of teachers, lots of classrooms, and that’s lots and lots of kids who don’t get to take the 3rd grade over,” says Daniel Weisberg of the NTP. “They only get one chance to learn how to read, or do fractions, or write an essay.”

The correlation between good teachers and development for children is solid, but there is still no agreement on how to create a good teacher in the first place. Charter schools, where teachers enjoy greater latitude, less bureaucracy and are typically not protected by unions, only outperform regular public schools 17 percent of the time. George W. Bush’s 2001 “No Child Left Behind” law enacted tough accountability standards, but made few concrete strides toward improving teacher quality. And that, ultimately, has to be the point.