Why wouldn't the argument apply to other pornography that encourage the fetish, like role-play? The internet is full of school girl role-play porn, porn that uses exceptionally young looking girls, "barely legal" porn, midget porn, father/daughter porn, "first time" porn, etc. I wouldn't even be surprised if porn built around this fetish was one of the most common porn genres out there.

If anything, a young-looking 18-20 year-old in a school uniform bring plowed on camera by her 42 year-old "teacher" is more morally suspect than some cartoon porn where every element of the scene is purely narrative and fictional. Especially since it's the former that would encourage actual child pornography (as amateur filmmakers or filmmakers in countries with lower ages of consent and/or policing push the genre further by filling the role with younger actors).