Yelling, getting angry, etc... are never going to add anything positive. This is pretty universally known.I am also a teacher.
In all of my years of teaching, never once have I yelled at a student. Literally not once.
I don't see any reason whatsoever to scream at a child, or any person at all, really.
Remaining calm and collected is a much better way to get the point across.
I agree with your point that there's a difference between screaming at a person, and loudly lecturing. The primary question is which one the OP employs.
But my real point is that increasing volume doesn't ever seem to actually help anything, regarding lectures/arguments/debates/etc.
However, not everyone has the same temperament, so while you find it easy to remain composed/calm not everyone does.
That doesn't mean yelling is correct, but yelling also doesn't make a person a monster or a bad parent.
The idea that everyone can be like you just isn't realistic, is my main point here.