Q. Why should a rabbi or a teacher who is very boring and the listeners fall asleep and are subject to unnecessary stress not be prohibited to lecture? Isn’t harming the health of the congregants and the class an issur (prohibition) of the Torah?

A. Indeed, there is a significant number of clinical studies that describe the stress and the harmful health issues created by people subjected to continuous boredom and constant tedium. These studies can be read on line.
However, Horav Shlomo Miller Shlit’a pointed out that people attending a drosho or lecture, are not forced to remain if they feel stressed. He mentioned that the most likely outcome would be the opposite. They would be likely to be rested and feel refreshed, when they eventually wake up at the end of the talk.
Lamentably, the same cannot be said on talmidim or students, especially young ones, who don’t have the option of leaving the classroom or constantly falling asleep. They are indeed forced to the harmful stress created by boredom, in addition to the crucial loss in learning the Torah and wisdom, expected by their attendance to yeshiva or school.
The Talmud (Pesachim 117a) relates that Rava before starting a shiur would mention a humorous point to rise the interest and joy in learning of his listeners. (See Divrois Moishe – Shabbos H, 80 on an extensive discussion on learning Torah out of simcha and joy).
Without doubt, it is the responsibility of the yeshiva or school to hire capable and professional rabbaim and teachers that will carry on with their essential and crucial commitment to teach properly their talmidim.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a.