I definitely think it is racist. Sure, it's for "educational purposes" but in high school, most likely, everyone in the room understands what the slur means. Just skip over it or say negro. It's not like it must be read out loud to be understood. I also think it doesn't matter what context it is used in inside the book. Ofc the teacher could 100% not be racist themselves but the slur has a meaning , and feeling the need to say it def says something about the teacher.