The School of Hard Knocks is an idiomatic phrase meaning the (sometimes painful) education one gets from life's usually negative experiences, often contrasted with formal education. It is a phrase which is most typically used by a person to claim a level of wisdom imparted by life experience, which they consider at least equal in merit to academic knowledge. It is a response that may be given when someone is asked about their education, especially if they do not have an extensive formal education but feel they have work/life experience that should be valued instead. It may also be used facetiously to suggest that formal education is not of practical value compared to work experience. In the UK, the phrases University of Life and School of Hard Knocks may be used interchangeably.
The University of Hard Knocks is a book by Ralph Albert Parlette (born 1870). It was first published in 1914 and was formulated from a very popular lecture given by Mr. Parlette in chataquas and lyceum courses. The book is available from Project Gutenberg as well as a number of commercial sites.
The idea that life experience or "training" from the "School of Hard Knocks" is equivalent to formal education is often exploited by diploma mills.