It is an English (the country, not just the language) way of speaking.
They also say that someone is "in hospital" rather than "in THE hospital".
But in the UK, one does not ‘graduate’ from secondary school at all (which is what Hogwarts is, after all). The term is starting to spread into British English because of the American influence, where it is a correct term, but historically and correctly in Britain, one only graduates from a university (which is why those studying their Bachelors degree are undergraduates).
So whichever way you look at it, it’s bad English :)
Personally I think ‘graduated from...’ is also better in British English regardless, and would see ‘graduated’ without the ‘from’ as more an American idiom.
Not to mention, that Harry did not complete his schooling at Hogwarts anyway (he left after his sixth year, not returning for the seventh, instead fighting the final year of the war that year) and according to JK Rowling, did not go back afterwards (nor did Ron, although Hermione, naturally, did).
And while we’re at it, what’s with this thing (increasingly prevalent I might add) of leaving out the verb “to be” in expressing infinitives?
For example people will say (and write):
“This food needs cooked.”
“The house needs cleaned.”
“The dogs need walked.”
All of these things need *TO BE* cooked, cleaned or walked.
Or, they might need cooking, cleaning or walking.
< /grammarnazi >
“...Two peoples separated by a common language.” —George Patton