It's quite common, particularly long after the fact, for teachers to say things like "of course we knew he was special". The Turing Scrapbook, which is at least partially biased in favour of Turing as a demi-god(!), makes no claims as to Turing being notably gifted as a child; indeed mentioning he was nearly stopped from doing his exams.

He was certainly a gifted child but his exceptional mathematical abilities only appear to have really taken root during his University career.