Is it possible to make it so that the site will not accept a post that has homework as the only tag to the post? I know that this problem only comes up rarely (as discussed here, but if we can automatically require that some other tag be added, it might help.
As I mentioned before, there really is no provision for this that wouldn't be a bizarre, contorted workaround.
If it truly comes up rarely, as you stated, perhaps human handling is adequate in this case?
If it becomes some kind of epidemic, we can revisit it.