A Los Angeles Times article this year recounted how a teacher rebuked an eighth grader who had been hospitalized for slashing his wrists in a suicide attempt. “Carve deeper next time,” the teacher allegedly advised. He was even said to have added: “You can’t even kill yourself.” A review board blocked the termination of that teacher.

The Los Angeles Times investigation found that it is so expensive to remove teachers that the authorities typically try to do so only in cases of extreme misconduct  not for something as “minor” as incompetence.

Of course, there are many other obstacles to learning: lack of safety, alcohol and narcotics and troubled homes and uninterested parents. But there’s mounting evidence that even in such failing schools, the individual teacher makes a vast difference.

Research has underscored that what matters most in education  more than class size or spending or anything  is access to good teachers. A study found that if black students had four straight years of teachers from the top 25 percent of most effective teachers, the black-white testing gap would vanish in four years.

There are no silver bullets, but researchers are gaining a better sense of what works in education for disadvantaged children: intensive preschool, charter schools with long hours, fewer certification requirements that limit entry to the teaching profession, higher compensation to attract and retain good teachers, objective measurement to see who is effective, more flexibility in removing those who are ineffective.

Unions are wary in part because school administrators can be arbitrary and unfair. Yet there are some signs that the unions are rethinking their positions in very welcome ways. The National Education Association has announced an initiative to improve teaching in high-poverty high schools, and the American Federation of Teachers is experimenting with teacher evaluation that includes student performance data.

Neither initiative reflects sufficient urgency. But let’s hope this is a new beginning. I’m hoping the unions will come round and cooperate with evidence-based reforms, using their political clout to push to raise teachers’ salaries rather than to protect ineffective teachers.

This is the central front in the war on poverty, the civil rights issue of our time. Half a century after Brown v. Board of Education, isn’t it time to end our “separate but equal” school systems?