You have highlighted a dilemma which can face professionals in many walks of life, who know about the law of copyright, but come under pressure from clients whose knowledge about such things is minimal.
It is worth differentiating between characters which start off in graphic form such as the Walt Disney characters, and those who are portrayed by actors in feature films. The former are fairly extensively protected by copyright in virtually all their poses, even if the new work has not been copied directly from any pre-existing image or scene from a Walt Disney film. The latter offer considerably more latitude under UK law, because if you were to draw a scene, for instance, featuring Harry Potter, Hermoine and Ron Weasley which did not actually occur in one of the films, then it is unlikely that there would be any copyright infringement because you would be using the likenesses of the actors Daniel Radcliffe, Emma Watson and Rupert Grynt which are not, per se, protected. That is not to say that, if they became aware of such a painting, Warner Bros who own the rights in the films, wouldn't try to object in a fairly robust manner. However they would no doubt be looking at the issue from the perspective of US law which goes much further in its protection of fictional characters than is the case under UK law.
You make the point that many artists just ignore the copyright issue, and they probably get away with it through the obscurity of their work, especially if it is a mural painted in a private home or school. That doesn't make it right, but it is easy to see the temptation of undertaking the commission and then disappearing to leave the institution to face the music if the matter ever comes to the attention of the copyright holder. Of course the legal solution is to get a licence, but as this would undoubtedly cost quite a bit of money, I suspect it is seldom done for one-off paintings and the like.
Of course many of the more well-known characters of the sort you mention will also be protected as registered trade marks. but I doubt if that is relevant in this context since no goods or services are being offered for sale in connection with the characters.
I'm sorry not to be able to offer you more practical help with this problem.