One example of segregation in A Lesson Before Dying is how Grant and other black people are forced to enter homes through the back entrance. That is the entrance that is reserved for servants (who in this case are black). Grant is not a servant, but is still forced to enter the home through the back entrance because he is black.
Another example of segregation is the school itself that Grant teaches at. It is an all black school. Grant not only teaches students of a certain age, but he is only teaching black students.
On top of that, Grant only is allowed to teach his students for roughly five months, since all of his students are forced to return to their field work at harvest time.
Another example comes in chapter 7 when the superintendent of the schools comes to visit Grant's school and students. It's a humiliating scene in which the superintendent checks the students' teeth and hair and gives them a big speech on hygiene. It's very reminiscent of the slave trade and slave auctions.
The reader then finds out that the funding for Grant's school is lower than that of the white school. His students get the old worn out textbooks from the white kids' school.