All he meant was that there had to be ways of keeping the kids in some sort of order (and smacking wasn't the best), but the connection between "discipline" and corporal punishment is too close in too many minds. "Whacks them then, does he?" you can hear them saying. It didn't help much when he saw "a clear dividing line between administering discipline on the one hand and violence on the other", which seemed to carry a discipline- equals-beating message, the only question being how hard one hit.
The word, as every schoolboy used to have it beaten into him, came from disco, I learn, and discipulus, a pupil, hence disciplina, learning or training, a sense which survives in donnish questions such as "What's your discipline?" meaning what branch of learning. Medieval teachers spent much of the day flogging their pupils, and discipline was no euphemism: they genuinely thought this was the way to teach. Most of them were monks or nuns, who believed in mortifying the flesh and imagined that what was good for them must be good for the children. In 13th-century monkish parlance discipline meant self-flagellation, which is the first recorded meaning of the English word. (The scourge itself came to be called "the discipline".)
Of course this is all very old hat and nowadays we prefer to discipline children in more civilised ways (imposing a curfew on them, perhaps). But the old connection persists. A visit to your local porno-outlet will confirm this. It's pretty sure to have a shelf labelled DISCIPLINE AND BONDAGE, and every "adult" knows what it means.