Graduated in 2004 and I still have a student account!!
My theory is that they let you keep the student account with no fees so you don't switch banks and just as importantly that you don't switch branches.
The bank managers of the student branches don't want to loose all the students that suddenly start earning money. From my personal experence and that of my friends, If your a recent graduate with a good income they won't change your account only if you request something outside of what the student account provides. e.g. on line share dealing
If they did start charging me, I would change bank accounts in the morning.