F-
The story doesn't do anything, it doesn't have any plot setting, any plot twists, it's a straightforward A to B tale.
The subject matter will also tend to lose you marks, examiners, teachers etc don't tend to enjoy reading work that contains excessive violence.
Your level of English seems to be good though, you could just use more imagination. To be truthful, this idea could do with being scrapped and you coming up with a new one.
If you are intent with sticking to this story, then do as sytaylor has advised and build it up a little.