Austin, that was horrifying. Ugh, vase shaped children!
Its probably apocryphal. The only reference we have is Victor Hugo. The infamous essay by Rand on Progressivism uses it as a metaphor.
But, the Chinese did do Binding and the Mandan did alter the shapes of their kids' heads. And then there are the Castrati, most of whom were Spanish. They were castrated and then trained to give them some amazing voices.
However, if one takes the school system as the jar and the child's mind as the body, it does seem apt.