Suppose your child is about to enter the fourth grade and has been assigned to an excellent teacher. Then the teacher decides to quit. What should you do?

The correct answer? Panic!

Well, not exactly. But a landmark new research paper underscores that the difference between a strong teacher and a weak teacher lasts a lifetime. Having a good fourth-grade teacher makes a student 1.25 percent more likely to go to college, the research suggests, and 1.25 percent less likely to get pregnant as a teenager. Each of the students will go on as an adult to earn, on average, $25,000 more over a lifetime — or about $700,000 in gains for an average size class — all attributable to that ace teacher back in the fourth grade. That’s right: A great teacher is worth hundreds of thousands of dollars to each year’s students, just in the extra income they will earn.

The study, by economists at Harvard and Columbia universities, finds that if a great teacher is leaving, parents should hold bake sales or pass the hat around in hopes of collectively offering the teacher as much as a $100,000 bonus to stay for an extra year. Sure, that’s implausible  — but their children would gain a benefit that far exceeds even that sum.

Conversely, a very poor teacher has the same effect as a pupil missing 40 percent of the school year. We don’t allow that kind of truancy, so it’s not clear why we should put up with such poor teaching. In fact, the study shows that parents should pay a bad teacher $100,000 to retire (assuming the replacement is of average quality) because a weak teacher holds children back so much.

Our faltering education system may be the most important long-term threat to America’s economy and national well-being, so it’s frustrating that the presidential campaign is mostly ignoring the issue. Candidates are bloviating about all kinds of imaginary or exaggerated threats, while ignoring the most crucial one.

Mitt Romney, who after his victory in New Hampshire on Tuesday seems increasingly likely to be the Republican nominee, refers to education only in passing on his Web site. The topic receives no substantive discussion in his 160-page “Believe in America” economic plan.

This latest study should elevate the issue on the national agenda, because it not only underscores the importance of education but also illuminates how we might improve schools.

An essential answer: more good teachers. Or, to put it another way, fewer bad teachers. The obvious policy solution is more pay for good teachers, more dismissals for weak teachers.

One of the paradoxes of the school reform debate is that teachers’ unions have resisted a focus on teacher quality; instead, they emphasize that the home is the foremost influence and that teachers can only do so much.

That’s all true, and (as I’ve often written) we need an array of other antipoverty measures as well, especially early childhood programs. But the evidence is now overwhelming that even in a grim high-poverty school, some teachers have far more impact on their students than those in the classroom next door. Three consecutive years of data from student tests — the “value added” between student scores at the beginning and end of each year — reveal a great deal about whether a teacher is working out, the researchers found.

This study, by Raj Chetty and John N. Friedman of Harvard University and Jonah E. Rockoff of Columbia University, was influential because it involved a huge database of one million students followed from fourth grade to adulthood.

The blog of the Albert Shanker Institute, endowed by the American Federation of Teachers, praised the study as “one of the most dense, important and interesting analyses on this topic in a very long time” — although it cautioned against policy conclusions (of the kind that I’m reaching).

What shone through the study was the variation among teachers. Great teachers not only raised test scores significantly — an effect that mostly faded within a few years — but also left their students with better life outcomes. A great teacher (defined as one better than 84 percent of peers) for a single year between fourth and eighth grades resulted in students earning almost 1 percent more at age 28.

Suppose that the bottom 5 percent of teachers could be replaced by teachers of average quality. The three economists found that each student in the classroom would have extra cumulative lifetime earnings of more than $52,000. That’s more than $1.4 million in gains for the classroom.

Some Republicans worry that a federal role in education smacks of socialism. On the contrary, schools represent a tough-minded business investment in our economic future. And, increasingly, we’re getting solid evidence of what reforms may help: teacher evaluations based on student performance, higher pay and prestige for good teachers, dismissals for weak teachers.

That, and not most of the fireworks that passes for politics these days, is the debate we should be having on a national stage.