When schooling is compulsory, schools are, by definition, prisons. A prison is a place where one is forced to be and within which people are not free to choose their own activities, spaces, or associates. Children cannot walk away from school, and within the school children cannot walk away from mean teachers, oppressive and pointless assignments, or cruel classmates.
The writer, Peter Gray, teaches at Boston College. He does not mention school choice, by way of vouchers, for instance, but it seems to me the freedom to quit is comparable to the freedom to choose one’s own school or other place of learning.