Okay, since the grading button was declined. How about a grading ban? An option teacher or the other adult staff would be able to use if they see a user that always grades wrong or however he likes or something? A small punishment where they would be able to enter the timeframe if they repeat their behaviour, they get maybe 1-3 hours grading ban and if it repeats again for a longer time.
That way people would try to make sure they grade correctly, so that they can continue to do it and get their money and points.
Customer support service by UserEcho