Did’nt you go to school in India?
Actually, I think the implicit assumption that the rest of the world must surely follow all the same arbitrary customs observed in the United States deserves a little snark.
It is a bit like being asked how we celebrate July 4th in England (which I have, indeed, been asked).
Well, at my convent school after we sat our GCSEs we had a presentation ceremony that family were invited to, where we were presented with our Record of Achievement (sort of a formal record of all the extra-curricular stuff we’d done at school, as we wouldn’t get our exam results for a couple of months), followed by a Mass.
I then left and did my A levels at a sixth-form college. No ceremony whatsoever there - but we did all get drunk together the day we collected our exam results.
Generally though, as others have said, graduation ceremonies aren’t common in UK schools.
It occurred to me that I didn’t know what Thai high schools do, so I asked the wife. No ceremony here either. There is for university, and that’s a very big deal, but oddly it’s held some months after the end of the school year. (First semesters ends about October and the whole school year about March or April; commencement ceremony in July.)
I was under the impression that the kids do have graduations and proms and other stuff imported from the US these days - so the assumption isn’t that snark-worthy.
Do they wait on grades? I imagine most profs would get grades in sooner than July, but some classes (seminars with lengthy papers, for example) might not, and the delay might be meant to ensure everyone walking is actually entitled to gradute.
The answer to which is, of course, “four months later”.
But really, every American should know that Independence Day is a distinctly American holiday, and that of course other countries wouldn’t celebrate it. But there’s no a priori reason to suppose that high schools would work differently, since after all, there’s far, far more that’s the same between Americans and Brits than there is that’s different. To take the opposite extreme, if someone asked “Here in America we eat by putting food in our mouths. What orifice do you put food in, in Britain?”, you’d be rightly offended, since there, the American should assume that we do things the same way.
And, of course, the fact that the OP is even asking the question in the first place is evidence that he’s not taking for granted that the Brits do it the same way. That would be something like complaining “Rowling isn’t a very good writer, since she never even bothered to mention the graduation ceremonies of any of Harry’s older friends. How could she possibly miss a detail like that?”.
No, it’s nothing like that. The grades are known almost right away. It’s just the way it’s always been done. I did know about the university practice, it was only for high schools I never knew, and I’ve always found the lag time for university commencement odd. The wife can tell me no good reason for it.
At many if not most or all universities and institutes here – and there are 159 nationwide – a member of the royal family hands out the diplomas. It used to always be the king, so maybe it’s a scheduling thing, but ill health prevents him from doing that now. The crown prince or one of the princesses will usually do it in his stead.
Thanks for all the interesting answers!
How many royals are there available for that?
Hard to say. There’s the crown prince and the three or four main princesses. But then there are some granddaughters, who are also princesses, and even an ex-wife of the crown prince, who is still a princess I think.
I don’t get it. 
The real answer is “only Americans celebrate wars that they lost”
I’ve had Americans ask me what I’ll be doing for Thanksgiving.
On the contrary. The OP isn’t asking about graduation ceremonies in Britain, The OP’s asking about a specific fictional institution, which in itself contains the assumption that the British hold such ceremonies.
As the United States is not the only country that celebrates Thanksgiving, it is not an unreasonable question.
Also, could you enlighten me to which holiday we have for a war the U.S. lost?
(Psssst: November 5 is Guy Fawkes Day in the UK. Fireworks ‘n’ stuff. More or less four months after July 4, though even we Americans do understand that the two holidays aren’t actually related. 
)
I’ve also heard July 4th referred to jokingly as “good riddance day” by an Englishman. But then, he was living in the US, and I think was just looking for an excuse to go to cookouts and fireworks shows.
School Balls, yes, definitely – There’s a ball at Hogwarts in Goblet of Fire; they had such things in NZ even when I graduated High School depressingly close to 30 years ago… although not necessarily quite along the lines of US Prom. (I do get the impression that modern school balls – at least at affluent NZ schools – are now more similar to the US Prom).
In NZ we’d sit exams in late-November, early-December… school (or university) would break up for Christmas holidays about mid-December. Around mid-Janary you’d receive exam results in the mail. 1st university term of the new year would start late-Jan / 1st of Feb (or something like that), and the graduation ceremony for those that had completed their degree would be April-May.
Not a holiday, but the National Anthem.
cite?
It’s so nice to know y’all remember Canada exists.
Other countries have Thanksgiving, as well - either imported from the US, or, like the US and Canadian versions, independently regularized versions of the existing festivals.