Answer:
Subtle racism when they're young
Step-by-step explanation:
NOTE: Please take my response with a grain of salt! I am merely answering out of thought, and what do I know? This is not the absolute truth, thanks!
I believe students in younger grades would be victim to subtle racism. I really don't think that in elementary/middle grades you'll be getting students coming right out at African-American (just using them as an example, by the way! :>) and saying that they dislike them merely on the basis of their skin color. I think it's just their own subconscious bias that students would experience, thoughts like, "They don't really look like me. I don't think I'm going to talk to them." or, perish forbid, "I'm scared of them. They look different.".
Even worse, would be racism on behalf of the teacher. Being skeptical of the student's intellect because of the place they live/part of the city they live in (not sure if this is just an American problem, but if you live in the bad parts of the city then most people assume your stupid), or maybe the teacher chooses not to call on them.
Not all racism has to be straight-out and obvious. Sometimes it's quiet, and micro-aggression's toward people.