The black hole of the American school system is the classroom. Researchers rarely describe and measure in a consistent or detailed way what is happening in those enclosures. It is usually expensive and time-consuming to get a representative sample.

That has long irked Larry Cuban, a former D.C. teacher and former Arlington County school superintendent who has become one of the nation’s most original and persistent education scholars. He has tried to take readers into classrooms. He has shown how little effect the policy and curricular changes we argue about — “New Math,” “New Social Studies” and computerized learning — appear to have had in those rooms full of desks, backpacks and children.

Now he has brought to life two high schools — Cardozo in the District and Glenville in Cleveland — as they were in the 1960s when he taught in them and also in this decade when he returned to gather material for his absorbing new book, “Teaching History Then and Now: A Story of Stability and Change in Schools.”

In some ways, for those of us long interested in Cardozo, it is a good news story. Cuban notes that the history teachers he watched at Cardozo in the 2013-2014 school year were more likely than those he watched at Glenville that same year to adopt “historical inquiry through primary sources to teach students critical thinking skills.”

He describes a lesson by one Cardozo world history teacher who taught the 1937 Japanese invasion of Nanking, China, by asking students to draw inferences from photos without captions and decide, working in groups, whether various unlabeled accounts were from Japanese or Chinese textbooks. He asked students who had not participated in the discussion to say which textbooks they considered more trustworthy and to note which sentences led them to that conclusion.

It is a prime example of what Cuban, professor emeritus of education at Stanford University, calls the historical approach to history teaching, questioning documents and analyzing ideas as historians do. Much of his book recounts the long battle between that approach and what he calls the heritage approach to history teaching, emphasizing the principles of American constitutional democracy, the wars fought for those principles and the need to cherish them.

I spent a lot of time at Cardozo 17 years ago, following around a motivated low-income student who, I was surprised to learn, was getting a pretty good education from several talented teachers. The school now has eight Advanced Placement courses, compared with none in the 1960s. It has a beautiful new building and an English teacher, Frazier O’Leary, who is nationally celebrated as an Advanced Placement instructor.

But Cardozo remains a school of mostly low-achieving, poorly motivated students from families with low incomes. In another history class in the 2013-2014 school year, Cuban watched four of the 12 students continue to chat rather than start the day’s assignment as they had been told to do. When one of the uncooperative students told the novice teacher to “leave me alone,” he did. The minute the teacher stepped out of the classroom to take a cellphone call, seven students stopped reading or writing and began talking to one another.

Educational theorists believe that getting students engaged in their studies by having them analyze documents, debate issues and explore the relevance to their own lives will help them learn more. But there is no significant proof of that at schools such as Cardozo. Cuban shows that the historical approach has so far not gotten far. “I estimate that between 15 and 25 percent of social studies teachers (including those teaching AP courses) have adopted versions of teaching history as an inquiry where primary sources are used at least once a week,” he said.

That’s an educated guess from a remarkable scholar. We need more information and more good teaching if we are going to take classrooms like those at Cardozo where we want them to go.