I think that the effect of such a societal description is very much contained in the question itself. You mention those included and those excluded, and I think those are key words. Representation is a very big deal, whether it is in education, media, or just day-to-day life. To hear a teacher describe society as primarily white, heterosexual males creates an ideal of who is important in society and who matters. To hear that you are not part of this would make you feel unwelcome and unimportant.
I think that education is hugely important for representation as well because we take it at face value. It is easy to think critically of art or media. For instance, it's common practice to walk out of a movie and immediately begin dissecting what was good or not. We are taught that media does not necessarily equal reality. On the other hand, we are taught that education is exactly true, always. This is how the Western education system works; if we question whatever we are learning too hard it will distract us from learning and make us not believe anything we are told. Because of this, educators or teachers providing a very narrow and problematic view of society is doubly troubling, as it is presupposed that it must be true because they say it is. This can have a major effect on confidence, self-image and a feeling of belonging.
Additionally, it may teach those that are included that they do not need to make an attempt to help or fight for the rights of those who are excluded, which can create a cycle of misrepresentation and discrimination. The moral of the story here is to fight for accurate and diverse systems of representations in not only media, but education as well. It can have a big change!