The text of the first Harry Potter book describe the school uniform as a black robe with a black cloak, a hat, and dragon hide gloves when hand protection is needed, and I think that's pretty cool. The movies showed a boarding school uniform with a robe on top, and I think that's pretty cool too, because it visually calls attention to the mixed genres of the series. What was the first official wizarding world work to have a Hogwarts uniform style based on modern school uniforms?
I know there won't be a clear line between fully medieval witch robes and full uniforms with trousers or skirt, sweater vest, collared shirt, and color-coded tie. I think that this can be approximately as exact as any other history-of questions.
In this collection of American chapter art, the first obviously school-uniform based outfit is in 2005 with a V-neck jumper, collared shirt, and possibly a tie. In earlier books, Harry usually wears a shirt with one wide stripe across, and I'm not aware of that being a school uniform look. It doesn't look like anyone else wears that type of shirt.
This French cover art counts, and if I'm not mistaken, it was printed in 2003.