In our current world, full of colorful distractions, an education system that relies on self-discipline is obsolete.
Some students have a "discipline" trait, they'll force themselves through boring drudgery to get the A, then never look at the material again, the A is all that matters. That's a waste of time.
True "discipline" comes from having an interest in the material. That interest has to be sparked by something, the material gives you a buzz, and working on it does not feel like work. That's not something I'd call self-discipline.