Technically (in the US), child pornography extends to pornographic material of non-minors who are made to look as if they are minors. This would probably included all of these school-girl fetishes and the pigtail stuff. Its fucking bullshit but... well, the law was written as a knee jerk reaction to public opinion.
Here is quote from the
text of the law (emphasis mine):
"advertise, promote, present, describe, or distribute the material in such a manner as to
convey the impression that it is or contains a visual depiction of a minor engaging in sexually explicit conduct. "