She’s not a plagiarist per se, but the idea of an elite school for wizards is not exactly original to her. There’s The Worst Witch series of books. The first one was published in 1974. They were a huge hit, especially in the UK, leading eventually to a TV movie with a very impressive cast list in the 1980s, which, you will note, was decades before Rowling wrote any Harry Potter book.

There’s absolutely no way she was not aware of those books. In fact, considering she was nine years old when the first one came out- the exact age for those books- she almost certainly read it and treasured it and it almost certainly inspired her to write Harry Potter and the Philosopher’s Stone.

On top of that, there was Ursula LeGuin’s Earthsea novels, the first of which takes place in large part at a boarding school for wizards where a poor boy who is from a non-magical family is sent after showing that he has magical powers and ends up being the most powerful wizard in the world, fighting the ultimate magical evil.

That was published in 1968 and I would be very surprised if Rowling hadn’t read it before she wrote her books, because that is just too similar.