I'd appreciate any help on a Year 11 High School assignment which requires writing a Modernist short story of no more than 2000 words. I have an idea of a story involving twin girls, one survives and one dies (falls off a cliff—I think). It would be written in the voice of the girl who's falling off the cliff about to die looking back at her twin. I'd appreciate any thoughts or suggestions.
You already seem to have a good basic idea in having the falling girl looking back at her twin sister. This seems to suggest using a stream-of-consciousness technique, possibly even going so far as to write the entire story in a single long sentence, like the thoughts of Molly Bloom in the last chapter of James Joyce's Ulysses. Since we are often told that a person's entire life flashes through his mind as he is dying, your story might include scenes from the lives of the twin sisters flashing through the mind of the girl who is falling to her death. This suggests to me that you might do something similar to what Ambrose Bierce does with Peyton Farquhar in "An Occurrence at Owl Creek Bridge." Farquhar's past life doesn't flash through his mind as he is falling with a noose around his neck, but in just a few seconds he fantasizes in vivid detail about escaping from the Union soldiers and making it all the way back to his plantation, where he is greeted by his loving wife. I suppose you would have to start off by somehow indicating that the girl is falling off a cliff. The girl might be thinking that she knows she is going to die but didn't realize it would take so long, that it feels more as if she is floating. Finally, it occurs to me that somewhere in the story you might specify that the girl is falling because her loving twin sister gave her a shove. But it's your story.
The main issue you face in completing the assignment is the single word "Modernist." In other words, simply writing a story with a contemporary setting won't suffice. The point is to show that you understand how modernist stories differ in themes and literary techniques from other types of stories.
The first thematic concern is the way modernism reacts to industrialized urban culture. Most modernist stories engage the notion of cities and the fragmentation of individual and cultural identity. If your twin were falling off a high rise building in a city and you gave some sense of how people reacted (ignoring her? photographing her with their phones but not helping?) that could engage modernist themes, but a rural setting might not.
Another major feature of modernism is that it breaks the illusion of its own mimetic nature. In other words, somehow the writer or narrator is intrusive in some way, making people aware that the story is a story, not a simulation of a reality. Often the narrators in modernist fiction are in some way untrustworthy or unreliable.
Finally, you need to think about how you can develop a plot. A conventional plot might have one twin save the other in a dramatic resolution, but that would not be a modernist approach. Modernism tends to subvert conventional notions of plot resolution often in a paradoxical way. For example, you might start the story with one twin dead and the dead twin talking about how she came to die, or you might create narrative ambiguity by emphasizing that no one could tell the twins apart and mixing their voices in such a way that we don't know which one is dangling and which one holding on.