@col-klink how about adding to Jill writing (and reading out) that Eustace has invited her to stay for the next school holidays and he's promised to introduce her to his cousins? (...when they go to London for a day trip or whatever...) 
Cross to scene with mother reading the letter aloud (joining in with Jill's voice, then continue with just mother's voice), commenting that she's glad that awful head teacher is gone, and father commenting that she'll probably go and make trouble somewhere else now - probably go into politics! Back to Jill, ending her letter with that she's glad they sent her to this school after all, and signing off. 

It feels like the sort of comedy bit that would work well as a series of mid and post credit sequences.

Inconsequential to the plot, but fun little extras for anyone sticking through the very end 

For what it's worth, the Head's fate in SC seems to be a toned-down version of a rather sharper satirical passage in the "Lefay Fragment", the beginning of a story that Lewis never completed, but that provided some of the basis for MN and a few other elements in the Narnia books:

Once there was a boy called Digory who lived with his Aunt because his father and mother were both dead. His Aunt, whose name was Gertrude, was not at all a nice person. Years ago she had been a schoolmistress and bullied the girls. Then she became a headmistress and bullied the mistresses. Then she became an inspector and bullied headmistresses. Then she went into Parliament and became a Minister of something and bullied everybody.

I don't see the Head's fate in SC as absolutely essential to the story, amusing though it is, so I won't be perturbed if it gets left out of the (hopefully) upcoming Netflix adaptation. The BBC didn't include it in their TV version. But if I were adapting all seven Chronicles, I would include a number of "in-between" bits that we don't see directly in the books — scenes in our world that show the Pevensies and Eustace and Jill and Digory (the Professor) and Polly starting to get together and calling themselves the "Friends of Narnia". Then perhaps in one of those scenes, there could be a brief conversation along these lines between Jill and Eustace — "Oh, did you hear what happened to the old Head of our school?" "Didn't she get made into an Inspector to interfere with other Heads?" "Yes, but it looks like she wasn't much good even at that. I saw her name in the newspaper last week — she's just gone into Parliament." "Oh, well, there's something that'll suit her at last!" (laughter)

That, to me, comes across as a natural way of doing it without being too much of a distraction from the story. I get the "show, don't tell" thing and agree with it in most cases, but occasionally there are times when it doesn't really work. If the film-makers switched to a scene where the Head is now an MP and getting into debates in the House of Commons, that could provide some good laughs for adults if it was written well, but it'd be above the heads of most of the younger viewers, at whom the story is aimed. I'd rather have it left out entirely than done in a way that doesn't suit the overall tone of the story. 

@col-klink    The only way I can think of is to end with Jill writing a letter to her parents explaining what's been happening at Experiment House (minus the Narnia stuff) which we'd hear as voiceover, probably over a montage of the events described. (If they did do that, I think it'd be cute to end the letter and the movie with her telling her parents she'd finally made a friend and for the last shot to be of her and Eustace.)

Much as I can see your point, I'm not really sure from my reading of The Silver Chair's ending that Jill Pole even had parents to write to. That may also explain why, at the beginning of the book, she was bullied at Experiment House. No parents to march in and complain to that Head, about just how their daughter was being treated. Though, having read Earl Spencer's memoir about his own private school education, two decades later, (A very private education), perhaps parents, in UK, weren't always very assertive about looking out for their children's welfare at boarding school, back in those post WW2 days, whilst C.S. Lewis, himself, hated his own pre-WW1 school. 

In the BBC audiotapes, though, the Professor is the narrator and maybe Jill or Eustace might well have written to him or have told him, instead. I'm off to check it out, anyway.