Tail-effects in education: Since interventions have to scale, they end up being mediocre to "what could be possible."

Related: Bloom's two-sigma problem:

Bloom found that the average student tutored one-to-one using mastery learning techniques performed two standard deviations better than students educated in a classroom environment with one teacher to 30 students

(haven't vetted the Wikipedia article or underlying research at all)

John Stuart Mill and his Autobiography are another source of relevant anecdotes for this question.