the "stoon" pronounciation is an alternative pronounciation used mainly by scottish people or ignorant english people who only know it as prince charles's school and assume that students are woken at 5am with enforced cold showers and then made to run several miles before breakfast and two hours of chapel. at the school itself, everyone uses the "stun" pronounciation. using the other identifies you either as a local scot who doesn't know/care much about it or just another ignorant englishman.