Hmm... Maybe we can flag certain links as containing nudity? Then, the link to an actual image would not display, nor would the image load if a person made a setting in their Preferences. Perhaps links to nudity could be restricted so that they can only be referenced by a certain subdomain, which schools could (probably) block fairly easily, regardless of a person's preferences. That's just an idea off the top of my head, I'm not sure it's any more feasible than flagging certain articles to contain boilerplate text like disclaimers.
This should be part of a general category scheme, not a specific solution for images which are perceived as possibly offensive to a limited segment of the population. What if someone finds articles about Christianity offensive? Can they block them, too? Singling out the issue of sexually explicit material is not neutral.
Regards,
Erik