I've figured that the best reason one might want to use the term "open source" instead of "free software" when talking to educators is to avoid the confusion between software that is "free as in speech" and proprietary code that is merely "free as in beer" (or the difference between libre and gratis). However, since there are two politically fraught terms that essentially mean the same thing, lots of people now split the difference: "Free and Open Source Software" or FOSS. It seems like an unnecessary acronym to me, but I can live with it. Increasingly, however, I see educators, who seem to be trying to be helpful, using "Free and/or Open Source" as encompassing "libre and gratis" software, rather than indicating two different ways of saying "libre".
This is bad, bad, bad, bad, bad. Do not do it and point it out to people who do. I don't care what you think philosophically about this stuff, but it is a misuse of language. Certainly there is built in ambiguity in saying "free software" in English, but "free and open source" has a specific meaning.