You're looking at it from the angle of restricting the viewing of content, and I am looking at it from the angle of restricting of posting of content. Two totally different things altogether, hence the different train of thought.
For the record... okay, perhaps this is the case.
But I’m saying it really does not make sense to ban the John Doe you speak of. (for reasons which you already stated)
It all comes down to the staff, there is no "save-all" method, which is what the OP is looking for.
If John Doe occasionally drinks alcohol, does that mean he should be restricted from eating at Burger King ?
If John Doe occasionally watches rated R movies, does that mean he should be restricted from watching a Disney movie ?
If John Doe occasionally posts (what you would say is inappropriate material) legally on an adult forum, does that mean he should be restricted from visiting his schools web forum ?
What does this have to do with what I said?
It’s not even what I said, nor what I implied.
I’m saying that schools have filters for THE kids that go to THEIR school!
It’s supposed to protect the kids from viewing OTHER sites that have potentially inappropriate content.
Not for other people to be restricted from viewing that school’s web-board or site.
According to the op, yes he should be restricted.
And I am saying he should not
And I agree with that.
It makes no sense to implement that type of system.
Now, there IS one alternative...
If you have good word filtering, that is going to be important, but to keep the content from being posted, you could implement some kind of system that is triggered by words that might be contained in a link (or image url).
This would cause the post that contains the image, link or other content to be withheld from public viewing (the Moderator Queue).
But for it to work like that, the OP would have to wait for some kind of MOD to be built for phpBB3 that would do that, but at least that would help a lot.
Just as an example, on STG, we have word filters on common spam terms, such as medications, and since those terms are usually contained in links, the spammers post ends up as http://[*SPAMMER*].com/[*SPAMMER*]/[*SPAMMER*].html (it’s really quite comical).
But you can use the word filtering to actually cause links to that kind of content to break, and therefore an image would not be displayed. -- Right now, I think that is the best option for the OP to go with.