That if one kid is poor or minority or unstable background, etc., and a bit of a loose cannon, they'll get graded lower for the same answers, punished or dismissed if they question anything, not going to be able or willing to check in before school or after class for more help, etc., meanwhile goody two shoes daughter of the town dentists will give the same answers, socialize better with the teacher, who knows the parent's status, and likes the kid, give her points back, and by the end of the semester they'll both be just as smart, but one will have an A- and the other a C+.
Basically kids having the "right" kind of parents and community connections and behavioral signifiers is at least ~1 letter grade overall regardless of students and ability.
The kids with sufficiently "decent" parents get handled with kid gloves and rule adherence, everyone else gets punched down and half-assery even when if they're experiencing injustice and unfairness.
Have known so many bright kids who get nowhere because of schools.