He did well enough academically that he made it into grad school in English at ASU. He was in grad school when he was expelled for refusing to see a psychiatrist about his mental problems and for chimping out in the office of Dr. Roger Lee, at the time the associate dean of judicial affairs.
School, at any level, is a highly structured environment, where you're told what to do, to what standard, and at what date and time to deliver it. All of the requirements and expectations are laid out for you, and if you want it there is plenty of institutional support, so if you are capable of doing the type of work required, and motivated to do it, you can get pretty far just by doing what you're told.
I've met plenty of people with advanced degrees who, to one degree or another, are functional retards who can't seem to manage mundane aspects of their lives, be it on a social or material level, or both. The "absent-minded professor" stereotype isn't as prevalent as it used to be, but it exists for a reason. These people are basically brains in jars, living intensely in their own mental world, while disconnected, to varying degrees, from the physical world around them (including their own bodies).
Basically, they have the ability to do academic work at an acceptably high level, while at the same time having significantly impaired executive functioning. Maybe it's ADHD; maybe it's autism; maybe it's some other form of brain damage; while some of it may be due to nurture, it appears, to me at least, to mostly inborn.
So it's not baffling to me at all how Sweet could graduate from college and get into a graduate program, while also being the kind of person who lets the exterior wall of his bedroom rot away over the course of years because he couldn't do something as simple as fix a broken window. I've encountered plenty of people just like him (though none with his pathological level of grudge-holding, nostalgia, and entitlement, which are why he's a cow with a thread here and they aren't).