The study was done in China, where making the class easier to improve pass rates isn't really a thing, as the Gaokao operates more like a stack ranking where it doesn't matter much how well you did in an absolute sense, only that not too many others who did better than you are competing for the same spot. Unlimited retakes are possible in theory, except each costs you a year of your life and except for some extreme cases of repeat test-takers, most people are going to give up after just one or two bad results. Homework is definitely not optional, but going home from school is. (Due to the hukou system, children often have to go to school far from where their parents work, so boarding schools with teacher-supervised self-study are common.) I don't see how holding back for individual classes is supposed to work considering scheduling conflicts.