Can somebody give me feedback on one of these five stories (they're each under 90 words... really short)?

They are interesting, because, like Lord of the Flies, they expose people's nature in a raw way. However, where Lord of the Flies was extremely verbose, this story's simple phrasing makes it seem as if a child is saying it. That is what you wanted, right? However, I think your Grade 4 story is the weakest, because, in all the others, something cruel or dishonest happens, but in the last one, nothing happens. You just elude to a past event, trying to sound wiser. All the other stories have the action happen. However, if you change it by writing about the tie you drank sour milk, it will seem like comeuppance or karma for the other stories when something bad happened. As I do not think this is the tone for which you are striving, you may want to select a different incidence to write about. I like your "simple diction," though.