From TFA, note that the 50% floor is applied per quarter, not per assignment. The effect remains the same, but is a bit more spread-out timewise. From my own experience, there's very little you can do for a kid who's discovered that there's no possible way to dig themselves out of their hole -- they shut down, act out, or just stop showing up. It bugs the hell out of me that we're playing with the numbers so dramatically, but I grudgingly grant its necessity for keeping edge-case students on board long enough to help them across. Further, I find that (in my own grading and that of my colleagues) it's ridiculously hard to earn less than a 50% with any degree of effort -- even the "coasters" shoot for the (passing) 60% mark each quarter. Higher grades aren't as easy. With a few worth-fighting-for exceptions, those who can't pull out at least an honest 50% probably won't be able to sustain a balancing 70% in the next quarter.