Beginning in infancy, black children are regarded as being stronger, more impervious to pain, less vulnerable compared with their white peers. From toddlerhood/preschool onward, daycare and teachers are more likely to be punished and punished more harshly than their white peers for exactly the same behavior. This pattern only escalates as the children grow older, throughout their school years and beyond. They are regarded as less intelligent, which in schools these days means less compliant and any small offense is seen as a larger offense than with a white child.
Reality: Back before the insane zero tolerance days history mattered. Yes, blacks often got punished more--because they had more of a record of wrongdoing. I'm thinking of the one time I got sent to the principal's office (I had been defending myself against an attack by some bullies.) The principal took one look at me, said "I haven't seen you before, you can go." "Same" action (fighting), very different outcome--because the principal applied some common sense and realized I was the victim. That's your "discrimination".