Minots claim that working students have little time for homework is faulty. In order for his claim to be true he should look at a group of students who make good grades and work and maybe take it from there. This is a good argument, but not a strong one that can stand on its own-yes, many can identify with the decline of study time when work time increases. Of course, Minot is making the assumption that a person needs to study a lot in order to make good grades and learn effectively. This is not true of everyone.
Students can obtain information at different levels.
I may need to study 3 hours to grasp a concept, others may only need 1, or they may catch on immediately. I especially like the ways you're locating particular claims of Minot's and debating their persuasiveness based on your own experience and reason. As long as individuals know what is best for them, they can make good grades. They just have to find out what is best for themselves. It does teach responsibility in the sense that you have to work and get all of your other stuff done as well.