I may have found another answer to add to the long list: Public Schools
I remember the health teacher in 5th grade putting a tampon in a small cup of water on her desk to show us how much they absorb. It flared out to like 5 times its size. I also remember watching these videos, probably filmed in the 80s, of a girl being embarrassed to bring a whole package of pads to school to put in her locker. Not only did that begin training us to be embarrassed, later when I was about high-school aged, a tampon product came out that (I believe) had the applicator plunger separate in the wrapper so it would only be 2-3" long instead of the 5 or so of other brands. They sold these little cases and marketed them as being more discrete than having to go to the bathroom during class with a crinkly pad or huge tampon.
This has a point, I promise!
Flash forward to today, being all passionate about cups, I decided to look up information on the health education curriculum from the public school district I went to. Clicking on the curriculum resources page, I found a link on "Puberty" in the "Family Life and Human Sexuality Unit"... Well, guess where the curriculum resource is from... Proctor and Gamble! Yes, the makers of Always and Tampax. No wonder we never learned about cups! Here is the a link to the site from the curriculum resources page: www.pgschoolprograms.com/puberty/.