double-breasted suits emit an 'old-man' vibe don't you think?
i'd go single-breasted. if you're tall and trim, go with a 3-button suit (that's me). if you're stocky, broad and not-so-tall try a 2-button suit?
as for suit colour, you might consider coordinating it with hair colour.
eg:
dark brown/black hair - darker suits
blonde hair - light/medium brown suits
personally i feel grey makes most 20-somethings look incongruous, grey (especially lighter shades) epitomize age (in a bad way). if you like grey go for a more intense and darker shade of grey. grey definitely projects seriousness though, good for conservative schools perhaps?
browns tend to give off a more relaxed vibe, perhaps for a semi-trad/modernized school?
haha i really do wonder how much suit colour figures in the interview. regardless of suit colour, being an effervescent and lucid conversationalist would do us good, no?