That's one of many many plot holes in Harry Potter.
There's really no depth to the world building beyond, "What if British public schools taught magic?"
It doesn't make sense in any context beyond that because the author never considered it from any context beyond that. Whenever you run into some crazy crap in HP and wonder, "Why TF would anyone do it that way?" The answer is almost always, "Because that's how they do it in British public schools.
It's also a bit odd that the spell so dangerous it was the absolute peak of the Dark Arts was a spell that just killed one person, and not even reliably. Where are the spells to call down meteors and cause earthquakes?
In the UK, "public school" just means that admissions are generally open to the public. They're still very expensive, are often boarding schools, and tend to have a lot of castles on their campuses.
It's not too bad when they use some word that we never use at all. I had no trouble figuring out what a "bellend" is.
It's more of an issue when they use a word differently.
Me: "Yo blondie, that's not how you're supposed to wear condoms."
Nigel Covington III: "You git, I most certainly do wear rubbers on my feet."
"Fanny" is my favorite. Both referring to anatomy, but one is on the back and everyone has one while the other is on the front and only one sex has one.