The "discipline" is not at all proportional to your son's "offense," and I'd be extremely upset at such an invasion of my child's person. I would want to talk with the principal immediately, with the teacher present. If that didn't meet my needs for clear communication, I'd go to the superintendent. I'd want that teacher to hear, clearly and loudly, that this is unacceptable on any level, and that it must never happen again to any student. I'd want the word of the administration that they will handle this in a professional manner, not by guilting a child into silence.
And having said that, I do think that proportionality is critical to a just and humane society. While some possibility existed for this ridiculous act to cause harm (I'm so glad it didn't), getting a person fired for a momentary, impetuous/unwise act is not proportional or reasonable. So I couldn't go that far with my demands.
While perhaps satisfying, punishment that outweighs misbehavior is destructive to civilization. In my 40+ years as an adult, I've been alarmed at how far society has swung into "accountability" that really is not accountable itself. And who knows when or how that could come back at us in hurtful and unexpected ways? All of us are perfectly capable of bad choices, stupid behavior, ignorance, or offensve mischief.
There's already so much punishment, as opposed to correction, for student behaviors that are little more than misdemeanors or refusals to bow to authority. Students end up labeled, in detention or suspended, even sent through the legal system, for doing things that would have simply earned a trip to the Dean's office when I was a kid. This teacher pumping a potentially harmful substance in a kid's ear for sleeping is an example of excessive "correction." Overcorrecting the teacher's unwise behavior would be perpetrating this pattern.
The only way we will find our way out of this mess in our society is to practice compassion instead of reactivity, and start forgiving. Correct, yes. Demand change, yes. Put the teacher under close supervision, absolutely. Fire her? Where does our own individual responsibility to create a compassionate world kick in?