How did Harry get his hair cut at Hogwarts?

Harry's hair is magically unalterable:

Harry must have had more haircuts than the rest of the boys in his
class put together, but it made no difference, his hair simply
grew that way
    _{Harry Potter and the Sorcerer's Stone, chapter 2, "The Boy Who
Lived"}

Aunt Petunia [...] cut his hair [...]. Next morning, however, he had
gotten up to find his hair exactly as it had been before Aunt
Petunia had sheared it off.
    _{Harry Potter and the Sorcerer's Stone, chapter 2, "The Boy Who
Lived"}

Harry [... had] jet-black hair that was always untidy.
    _{Harry Potter and the Chamber of Secrets, chapter 1, "The Worst
Birthday"}

His jet-black hair, however, was just as it always had been —
stubbornly untidy, whatever he did to it.
    _{Harry Potter and the Prisoner of Azkaban, chapter 1, "Owl
Post"}

He raised his hand automatically and tried to make his hair lie flat
“You’re fighting a losing battle there, dear,” said his mirror in a
wheezy voice.
    _{Harry Potter and the Prisoner of Azkaban, chapter 4, "The Leaky
Cauldron"}

Mrs. Weasley was attacking his hair with a wet comb. She pressed hard
on the top of his head. “Doesn’t it ever lie flat?” she said
desperately. Harry shook his head.
    _{Harry Potter and the Order of the Phoenix, chapter 7, "The
Ministry of Magic"}

Since cutting his hair useless, there is no point in doing so. So he (probably) does never get his hair cut at Hogwarts.

That would be a problem if his hair grew naturally, because it would become extraordinarily long. But that doesn't happen. So we can conclude his hair doesn't grow.

That means that, even if you cut it or let time go by, Harry's hair remains always the same.