Similar happened to me (UK) - my school went from performing under the national average for the preceding three years to being notably above for my year group,

Similar happened to me (UK) - my school went from performing under the national average for the preceding three years to being notably above for my year group, and no one could remember anyone going to Oxbridge, but there were two of us in that year (the school barely even knew about the extra exams we had to take - required for one, optional for the other - so the prep. was more or less entirely self-taught).