One finds in America that men who were idle as undergraduates work hard in the law school or the medical school, because at last they are doing work which strikes them as important. That is the essence of the matter : make the school work seem important to the pupils, and they will work hard. But if you make the work too easy, they will know, almost by instinct, that you are not giving them what is really worth having. Clever boys and girls like to test their minds on difficulties. With good teaching and the elimination of fear, very many boys and girls would be clever who now seem stupid and lethargic.
Source: On Education, especially in early childhood, 1926
More info.: https://russell-j.com/beginner/OE16-030.HTM