Hey this happened to me too.
My highschool was badly-managed, notoriously so. Some teachers weren't allowed to grade tests "for budget reasons" (We got a brand new locker room for the football team that year. And a new one two years after that)
Anyway, how do the school administrators make sure tests get graded? Student graders, of course!
Specifically ones who need a score boost in the same class they're grading.
And as it turns out, more often than not, ones who have beef with anyone who does better than themselves.
So guess which students suddenly jumped from 20% grade averages to 99% and which students fell from 79~98% to 10%.