An interesting facet in all pornography is heavy reference to young women still in educational institutions (college, high school, etc). The best example is Japan, namely anime porn.

I suspect the continual fascination with schoolgirls is because as teenagers many men had their first sexual fantasies. That is normally in high school, which is seen very frequently in the hentai (XXX anime) animation in Japan, or in college (the American version, being "hot college co-eds").

In porn, when watched by men who had many fantasies as boys raging in the hormonal storm, they would be able to connect with the schoolgirl depiction very easily with their own desires. Hence explains the coeds / plaid uniformed schoolgirls seen so much in porn today.

In my dorm right now, floorcest is very common. I think my floor has more sluts (sorostitutes / very sexually active girls) than any other floor in Cornell University. But except for one case, there is no participation in pornography that I know of (and that one case is not deliberate).

The co-ed/schoolgirl depiction in pornography is there to connect with the teenage fantasies inside our perverted minds, and cause more sales in schoolgirl porn. It works amazingly well.