Having just watched a film called The Club (1994), I'm pretty sure it's the one you were looking for.
Time stops at midnight at the Senior Prom for five students, one murderous counselor, and John. They must find the courage to face themselves or, when time starts again, they may find that they are joining John's Club. All you have to do is commit murder... or suicide.
The film begins with a high school's Senior Prom being held in a castle, replete with suits of armour and a guillotine in the basement. There are many people in attendance, mostly wearing medieval-themed costumes.
We get a few scenes introducing us to the main characters and establishing that some of them already have issues. For example, we see a girl breaking up with her boyfriend, who responds by becoming aggressive, as if he's about to hit her, before another student walks in on them, enabling the girl to flee the situation. The girl in this scene has curly hair and is wearing a black dress and a cloak, which fits the description of one of the characters mentioned in the question.
About 13 minutes into the film, the clock strikes midnight, and most of the people in the castle vanish, leaving behind six students. Two of them are girls, one being the girl in the black dress I already mentioned, and the other, a blonde girl in medieval garb. The four guys include the first girl's recently-dumped ex. The guys are mostly dressed in medieval garb too, except for a nerdy boy with long hair who's wearing a suit that looks a bit more modern. He could be the one you said was dressed like a '20s newspaper man. In fact, this boy mentions that he was the editor of the school paper last year, although I don't know if his outfit was meant to tie into that.
The Club (1994)
We see the kids wandering around the castle, trying to figure out what's going on. Around 23 minutes into the film, the girl wearing the black dress is in a restroom washing her hands and face, and when she looks up at her reflection in a mirror, she sees that one of her eyes is swollen shut, as if she's been punched. She runs back to the other students in a state of distress, but her face is fine; it was a hallucination.
The Club (1994)
Around 35 minutes into the film, the boy with long hair and the two girls find a room containing stacks of books, apparently all different volumes of the same title. They start leafing through the pages of some of the books, and see photos of various people's faces, along with numbers indicating the years in which these people were born and died.
Around 43 minutes into the film, the boy with long hair is alone in the room with the books, when a doppelgänger of him walks up and starts taunting him. He tries to run away, but the doppelgänger keeps following him, and seemingly uses some form of telekinesis to place him in restraints. The doppelgänger then claims that the boy had previously intended to commit suicide, and starts doing things to injure himself, such as slitting his own wrists. Corresponding injuries appear on the boy as well, and he howls in pain.
The Club (1994)
Then, about 49 minutes into the film, we see the same boy -- no longer in restraints, but with his eyes glazed over, as if in a trance -- placing his own head in the guillotine and pulling a rope to release the blade. Moments later, a picture of his face appears in one of the books, along with the years of his birth and death. The book then closes by itself, and we see that the title on the cover is The Club: Suicides vol. 1993. (There's no blood or gore in this scene, incidentally; the beheading is implied, rather than directly shown.)
The Club (1994)
I'll stop here, as I've matched all the details mentioned in the description at this point, so there's no need to spoil the plot any further. The film is available to watch for free on Tubi in the US and YouTube in Europe.
