One important thing to keep in mind about users encrypting their own messages: those encrypted messages will be impossible to discern from random bits.
This means if I send the message "f734fca50971f301786eaf0ee0de900f" there is no way to tell if I'm breaking the law or not unless we also outlaw sending random bits.
Sure, the course grades are not curved, but the students are definitely competing academically, especially those in the same class. There's class rank, honors status, merit scholarships, internships, job opportunities, etc., that are technically at stake. It makes sense to choose 6 every time.