It would be simple i reckon, SImon & Jay decide to go to the same University, lets say Warwick. Due to Will messing up his exams when he crapped himself, he doesn't get the grades to go to a better Uni,l therefore has to settle for his 'last choice' Warwick. As for Neil, he doesn't want to be left out, so he moves up to Warwick and gets a job in the University canteen.
Of course, it's a bad idea since they would no longer be Inbetweeners >_<
Uni doesn't really have a defined line between social groups like you get in school, it's more to do with what societies/where you live that determines your social patterns for the next 3 years.