Earlier quoted context omitted.
"My defiant and sarcastic reactions to accusations were often considered evidence that I'd done wrong when I was innocent." You were rebellious. From the point of view of a common teacher, that means you are not innocent. There is actually a old common saying, when some kid got a beaten, but it later turned out to be not guilty of that ... "well, he deserved it anyway" or "well, then the beating was in advance for so…
Having been through army basic training, I can confirm this is not an attitude unique to school teachers. To be fair, discipline and self control is incredibly valuable even if it’s not the be all and end all.
It is!
But you don't need to break someone, to help them learn it. Breaking people is just the cheapest method.