Fig. the school of life's experiences, as opposed to a formal, classroom education. I didn't go to college, but I went to the school of hard knocks. I learned everything by experience.
The practical experience of life, including hardship and disappointments. For example, A self-made man, he never went to college but came up through the school of hard knocks. This idiom uses knock, "a blow," as a metaphor for a setback. [Mid-1800s]