I learnt my theory as a child alongside classical piano lessons, but I've always followed the very basic rule that if a note's got a # on it, you just play the next note up. It seems to work quite satisfactorily. I don't believe any teacher would actually make the claim that B# doesn't exist, but nevertheless anyone already capable of reading a 'B' could work out how to interpret a B#, should they encounter one, without a great deal of thinking, special training or knowledge.