Absolutely correct. There is no doubt that more diversity has led to more understanding, but I believe that diversity alone has not solved the underlying problem of minorities being disadvantaged in even diverse educational settings. To give you a concrete example of the reality for minority students, despite diversity attempts--for students who enrolled in college full-time in 2005 and graduated by 2011, 69% of Asian-Americans graduated, 58% of Whites, but only 46% of Hispanics and 45% of Blacks. What that suggests is that, although some level of diversity exists in many schools, there is still a real deficit in educational achievement among Hispanics and Blacks that exists despite diversity. We can, of course, debate the reasons for this lack of achievement, but the fact is, it exists.
Still, I believe you are right to argue that diversity is an important element in education--there are both tangible and intangible benefits in diversity. Those benefits, however, need to be offset against the problems that diversity has been unable to solve.
As your question implies, diversity in the high school classroom has been proven to be a critical element in a public high school student's ability to learn at the highest level. For the last four decades, educators have been aware that ethnic and economic diversity among students has had a positive effect on the learning experience and, conversely, the lack of diversity has often resulted in a less-than-optimal education for students, especially for students in racial minority categories.
Complicating the diversity issue are studies that have shown that diversity alone does not necessarily lead to better educational outcomes:
Although a classroom may include diverse students, if some students perceive barriers and do not feel included, an actual atmosphere of diversity may not exist. ("Literature Overview: Diversity, Inclusion, and Cultural Awareness for Classroom and Outreach Education," page 1 [see citation below])
This 2012 study points out that Black and Hispanic students often discover that, despite diversity among the student body in public high schools, there are barriers to their ability to take classes from the best teachers, especially in math and science classes. The sad reality is that students are placed in the highest-level classes, taught by the most-accomplished teachers, based on the students' achievements in earlier classes. If minority students have not achieved top grades in basic-level classes, they are relegated to lower-level classes as they move through grades 9-12.
Racial diversity in public high schools is still, after years of attempts to create a balance in ethnic composition, undoubtedly the problem that results in a second-class educational experience for minorities. As a recent study has concluded,
The typical white student in the U. S., for example, goes to a school where roughly three-quarters of his or her peers are also white, even though whites now account for just more than half the national school enrollment. (see citation source below)
Despite attempts to integrate public high schools, the evidence shows that segregation in education still exists, and as long as schools fail to achieve true diversity, those who are segregated are likely to receive an education that reflects their minority status--even though, as demographics now show, that minorities are now achieving majority status in several areas of the country. It is a great irony of race that minorities who may actually represent the majority in numbers are still relegated to minority status in educational opportunities.
In addition to diversity issues with respect to race, several studies have pointed to different treatment of males and females in the classroom, a diversity issue that is subtle and difficult to address. A 2000 study, for example, indicates that teachers engage more often with male students, with the result that male students may "be challenged to use critical thinking skills" much more often than female students. (Brickhouse et al., "What kind of a girl does science? The construction of school science studies." Journal of Research in Science Teaching, 45(8), 955-970).
In sum, then, the lack of diversity creates an imbalance in educational opportunities for racial minorities, particularly Blacks and Hispanics. At the same time, however, as many studies have shown, statistical diversity still does not necessarily solve the problem of diversity. If students perceive, despite diverse classrooms, that they are treated differently, diversity itself is not solving the essential problem of unequal education. Diversity is certainly a step forward, but that step is not sufficient to create classrooms in which all students are treated as equals. And as several studies have concluded, male/female diversity problems also result in unequal education.
I see your point but isn't it possible to look at it from the point of view that the majority of students would benefit from a more diverse student body? I feel it would better prepare them for the "real world."
Coming from a high school that was 100% white (we had absolutely no students of different backgrounds... partially due to the fact that there were only about 150 kids for all four grades, and partially due to the small farming community in the middle of nowhere in which a majority of the jobs were 45+ minutes away), I believe that diversity in public high schools is important, and could lead to a better understanding and acceptance of other types of people. When I came to college, I was in for a huge culture shock. There were so many different types of people there, and I did not know how to act. I think that had I gone to a public school with students who were racially and sexually diverse, I believe that I would have gotten a better education; not a better textbook education, but a better real-world education, and I would be better prepared to accept others and coexist with different types of people in the real world.
Hope this helps!