When it comes to the history of slavery in the U.S., the central role it played in shaping the country and its continued impact on race relations, students don’t know much.

In fact, only 8 percent of high school seniors can identify slavery as the central cause of the Civil War, according to a report released Thursday by the Southern Poverty Law Center.

But don’t blame the students. As the report shows, schools aren’t including it in curriculums, teachers aren’t prepared to teach it and textbooks don’t include enough material about it.

“It’s important that everyone understand that slavery is truly at the foundation and formation of this nation,” Hasan Kwame Jeffries, associate professor of history at Ohio State University, said on a press call the center organized Thursday. “You really can’t understand the American past or present without having a good grasp of the role that slavery played in this country.”

But the report shows that two-thirds of high school seniors don’t know that it took a constitutional amendment to formally end slavery, and fewer than 1 in 4 students can correctly identify how provisions in the Constitution gave advantages to slaveholders.

“A lot of our students don’t even understand the role of the North in slavery and how it benefited from slavery,” Jackie Katz, a U.S. history teacher at Wellesley High School in Massachusetts, who was also on the call, said. “When they learn about the Civil War, they like to think that we in the North are the good guys. Massachusetts was the first state to abolish slavery. But then they hear about Lowell mills and they think, ‘Wait a second, where were getting that cotton.'”

The survey, which was conducted over the course of one year by the center’s Teaching Tolerance project, also found that teachers, textbooks and state academic standards aren’t helping the situation.

While teachers overwhelmingly responded that they feel comfortable discussing slavery in the classroom, the report found that nearly half think their textbooks are inadequate and 40 percent believe their state offers insufficient support for teaching about slavery.

“It’s hard to discuss violence and teach white supremacy. It’s hard to learn about the shortcomings of our American icons and heroes,” Jeffries said. “It’s hard to wrap our minds around the fact that something so vile undergirds our history. So we have tended to shy away.”

Popular textbooks, the survey shows, fail to provide comprehensive coverage of slavery. Of the 10 most widely used textbooks, researchers at Southern Poverty Law Center found only one that scored a 70 percent against a rubric of what the they consider should be included in the study of slavery, but the average score was 46 percent.

Moreover, the survey shows, states fail to set high expectations for students learning about slavery as outlined in their content standards: Of the 15 sets of state standards the center analyzed, none address how the ideology of white supremacy rose to justify the institution of slavery, most fail to lay out meaningful requirements for learning about slavery, about the lives of the millions of enslaved people, or about how their labor was essential to the American economy.

Indeed, Katz, who’s been teaching in Wellesley schools for 11 years, originally didn’t include slavery in her U.S. history curriculum because Massachusetts’ academics standards didn’t require it, she said.

“But when you want to teach about contemporary issues,” she says, “it all goes back to slavery and the foundation of the nation.”

Students can’t possibly grasp the gravity of the Civil Rights Movement or the genesis of the Black Lives Matter movements, she said, without being grounded in the history of slavery.

The report offers recommendations for states, schools, teachers and textbook publishers about how to better integrate the teaching of U.S. slavery into curriculums, which researchers at the Southern Poverty Law Center underscored as urgent for students today.

“If we are to move past our racial differences, schools must do a better job of teaching American slavery and all the ways it continues to impact American society, including poverty rates, mass incarceration and education,” Maureen Costello, the director of Teaching Tolerance and former history teacher, said on the press call.