I just completed the Grade School exercise and was initially puzzled about how the top community solutions were ensuring that the same student name could not be added to more than one grade. It turns out they don’t: even though they’re marked as correctly solving the latest version of this exercise, the relevant tests fail when I run their code locally. I suspect that constraint was added in a more recent version of the exercise.
Not to dunk on those solutions (they were more elegant than mine and taught me about BTreeMap/BTreeSet/BinaryHeap), but this caused me a bit of confusion.