Poindexter is still a 17-year-old, stuck in the retro spirit world version of Casper High. Only one of two things could explain this. Either the spirit world version of Casper High is a sort of purgatory for him after he died, which seeing as how if the episode took place in the 2000s when it was aired, he'd be in his 60s, which is kind of young to die. Or, he died as a result of a prank gone too far. Given the students who tortured him subjected him to hard physical torment with their pranks, including shoving him into a locker 1/3 his size, it's not hard to imagine one day he got shoved in there and couldn't get out (maybe over a holiday break) and died of starvation, dehydration, or even suffocation if the lockers didn't circulate air well enough for him to get necessary amounts of oxygen. Why else would his locker not have been assigned to another student in 50 years? It was abandoned for long enough that the door rusted over, with no sign of a repaint, and the mirror's still in it for some reason. Also leads to the question of if the locker was already haunted, or the school though it was cursed or something because of that.
A secondary one arises where, if the second one's true, why didn't his parents come looking for him when he didn't show up at home at the end of that school day? This would seem to imply they didn't care for him, either.
A third is Pointdexter actually committed suicide as a result of being depressed from the constant torture basically everyone at the school forced him to endure rather than being left in the locker. If this is true, then the Casper High he's stuck in is a Purgatory of sorts, and he's constantly stuck there as a spirit unable to move on doing the same things day in, day out for all eternity.
A fourth possible one is the fact that his classmates were in this purgatory with him. Did Casper High have some sort of tragic accident, like a fire? Or did Sidney actually perform a school shooting to get revenge on his tormentors?
