I read somewhere in Kasparov's book "How life imitates chess", something along the lines of "there are many things which you cannot be taught but that you can teach yourself". I think this is particularly relevant to people that struggle at school. I'd draw one corollary from this as applied to OPs question: as soon as you can turn the student into a teacher, tasked with working out how to teach themselves. It is a meta concern which exponentiates the value of a tutor.