You think a Down’s Syndrome child is going to behave logically?
If the skilled substitute had discerned that the child could be reasoned with, a negatively corporal consequence for misbehavior would not have been needed.
The consequence was merely treatment with a soapy paper towel, not a bar of soap, was not a very painful or harmful result for a not very serious infraction. But the clue of a rebellious attitude of the almost-adult child striking out at the instructor was a much more serious irresponsible behavior, invoking a reasonably equivalent escalation of the negative response from the teacher.
And in retrospect, the child's mother had not learned of her own deficiency of training her child to be obedient to justifiable reproofs from an appointed authority.