If a 50 can keep them statistically in the game, with a chance of turning it around and passing, that might be worth it. It's similar logic to not giving life sentences. People with no hope of a good outcome are harder to deal with. A kid with a 22% average that you have to deal with for 12 more weeks must be a nightmare. They have no incentive to try at all, or to let the class proceed in an orderly wat.