I don't suppose Josepp has considered the idea that maybe the reported comment of the teacher actually included something like the phrase,
" In the key of C major......."
In most keys there is no B sharp. I make it two out of twenty four.
So if the teacher was teaching scales, he or she was mostly correct, and it would be particularly applicable to the usual keys that a guitar player uses.
When did you last start a tune in F sharp or E flat minor without a capo?
Cheers
Dave