I've often thought that there's a potential for an interesting character dynamic with a character who's neither a Terry Long nor a Professor X - when you have older characters spending lots of time hanging out with younger characters, it's usually because a) the older one (almost always a man) is dating one of the younger ones (almost always a woman), or b) the older one is a platonic mentor to the younger ones.
One of the things I liked about Peter Parker as a teacher was that, when done right, it ironically emphasized his youth - "I'm supposed to be teaching these kids, who look up to be as an adult, but I feel like I'm barely older than a kid myself!" - so a Terry Long character almost could have worked, if he was in his early 30s instead of ... what, his 40s? And if he hadn't been fucking Donna, but instead, was somewhere between an equal peer and a mentor to these kids. Of course, he'd still need a reason to be there, but anyway ...