What a tough situation. It would probably be easier if it were coming from a parent, and Slick's approach would work.
Talking directly to a child though - and I'm not sure how old the kid is - you have to be a little careful. It's never clear how much is coming from the child or from the parents. I think the straight forward approach (a modified version of Slick's) is the best - IMO - talk to them about following rules, and that while you aren't in the class to see exactly what happens, that this teacher is known for being strict about following the rules - and that race/religion/gender etc. aren't factors - just whether or not the rules are followed. You might also add in that it's very hard for someone to evaluate what's really happening as an outside third party, and if the students are feeling like there is a problem they shoudl address it directly. That's where you have to be careful though as you don't want to imply the kids are lying, which they may not be - but they may just seeing the level of discipline and drawing conclusions.
Sadly, there are a lot of people in positions of power who *do* make evaluations and treat people - children or adults - very differently depending on their skin color. Or religion, or sex, or sexual preference, or whatever. It's nice to have an opportunity to influence things the other way, and you're in a good position to do just that
Did that make any sense at all?