They Bite by Anthony Boucher is like four pages long and had me jumping at every shadow in the corner of my eye for a week. I found it in my grandparents' copy of Alfred Hitchcock's 30 Best in Horror or something like that, bought a copy for the brother I like because it shook me so badly (I verified it was in there)

My freshman college English prof assigned House of Leaves.

It was awesome watching the preppy kids descend into madness

The Dweller in the Gulf by Clark Ashton Smith.

We had to read 'Der Vorleser' in which a 15 year old boy gets into a relationship with a 36 year old woman. A strange choice to force kids about that age to read (we were a bit older than 15, I think. But still...)

Of mice and men

"Nachts schlafen die Ratten doch" still haunts me...

"The Long Rain" by Bradbury was the one that stuck with me.

Maybe not disturbing enough, but the short story that really stuck with me was: https://en.wikipedia.org/wiki/Lamb_to_the_Slaughter

Blood Child ild by Octavia Butler. Humans living on an alien reservation have the males implanted by the insect like alien's eggs and they start burrowing out of your flesh when they're ready.

I remember in high school our text book had some paragraphs from various literature books. One of the books was called zombie (or zombies) so of course I checked it out, even if the teacher skipped it. The section was just a description or something, nothing particular, but I decided to borrow the book at the library anyway, and the full story was basically (spoilers ahead, it's gory):

I wonder if somebody did it as an Easter egg or what

https://en.wikipedia.org/wiki/The_Plague_Dogs_(novel)

go away now

The Book Thief by Markus Zusak

Hardfought, by Greg Bear. Sci-fi set in the far future, spoken with a military patois that is difficult to understand but is meant to highlight the alienness of the forever war that the story takes place in. Themes upon themes fifteen-plus layers deep, even though this is only a novella.

I have something north of 3,000 volumes in my library, and if I was to pick the most influential fiction story of my life, this would be it. I had difficulty reading it as a teenager who was typically reading at a university level while in high school, so it’s going to take serious effort by most to truly benefit from it. But when you finally understand those themes… holy shit.

Come and See by Soviet Union

It isn't a short story, but it might as well have been.

In my high school senior level English class, they had us read "On the Beach." The class as a whole did not like it. We told the teacher that we would not be reading further and would not be engaging on the book any more. It took a week and they moved us on to "Wuthering Heights" which was far easier to read.

The cold equations