The meaning has nothing to do with a real prison, what Victor was trying to say is he who opens a school door, closes the doors to a prison of ignorance. People who are not educated or miss a great deal in life, even though nobody can know everything, there are the basic things that most people assume everyone knows. For example some people don't know simple things like reading, math problems, and solving simple challenges in everyday life. Anyone who does not know these things is forever in a prison of his own making. Those kinds of people have made their own prison because they do not know the basic knowledge you need for everyday life, and will be forever cut off from enjoying a lot of what life has to offer. That is the prison Victor was describing and wanted us to turn away from, and he was right.