In my opinion Dr. Slawter is most likely to be the next Head, if we're looking for a 'good' kind of principal. He doesn't seem to like the Preps, Harrington(s) in particular, and thinks that success is all about connections, rather than one's personal abilities. He also complains about just how run down and corrupted the school is in general. He'd make a great goody-two-shoes kinda Headmaster if he ever adapted an 'im totes gonna clean dis schoo' kinda attitude.
But let's be honest, nothing's more likely to be the next principal than Money. Someone rich like prefect Edward or Hattrick. I'm including him because the whole Hattrick vs Galloway isn't exactly part of the main storyline, and Cheating Time can be done after Final Showdown, or not done at all. He's got connections with prep parents, he's corrupted as fuck and lives in a nice big house with a neat backyard. Not to mention his unpleasant attitude. But, he's also strict as fuck and probably has a thing for punishing people. Headmaster material right here.
I'd even consider the ever-so-incredibly-rich-as-fuck Mr. Smith to be a candidate, seeing as how he 'made this goddamn town' and is on his way to retirement. If I was some aging guy with more money than I can count and nothing to do, I'd go for it.