Evidence suggests that the race and ethnicity of a teacher, when aligned with the students, proves to be extremely beneficial to the students. The research shows that ethnic minority students do much better when they are being taught by a teacher of color, and from this many policymakers took a logical leap and associated most of the diverse teachers to schools of high ethnic minorities in an effort to steer better grades for the students.
The segregation of teacher based on color was done on singular evidential data that grades seem to approve when in alignment with race or ethnicity, however, didn't include the pro-social benefits that white children get when taught by a diverse teacher. Diversity in the workforce is extremely important, and just focusing on test grades as the marker on who should teach where dilutes the whole concept of diversity.
The benefits that are to be gained from a teacher can't be gained when such kind of segregation is taking place in schools. Imagine the impact on white children if they are never taught by a diverse teacher throughout their schooling life, the acceptance of diversity would be low. Sure test grades matter, but the evidence can at the end of the day be a correlation rather than causation.