what are the key features to get a high level mark in creative writing

There is no formulaic method of acquiring success in a creative writing class; for one, expectations will vary significantly with the academic level of the class. Also, depending on the class instructor, there may be several different grading criteria set out that determine the expectations of the class. Furthermore, writing, like all art, is subjective, which makes grading or marking any work inherently difficult.
That being said, there are several key factors in academic creative writing that one should always pay attention to. Poetic and powerful prose, psychologically complex characterization, strong dialogue, and, of course, the use of literary devices (metaphor, simile, portmanteau, onomatopoeia, et cetera) are all important traits to include in your writing. Depending on the type of class—that is, the specific genre of creative writing that is being taught—you might feel free to heavily experiment, adhere to a specific style, sketch out a satire, or many other things.
Pay attention to what stories, if any, are read and discussed in class. What stories have you read specifically that work? Which stories or novels have you enjoyed? Using those as blueprints, create something that you would like to read, but remember that your instructor will really be the one reading the piece.
Most of all, do what feels natural. Make sure you let yourself progress with each piece you write, for instructors typically aim to find improvement in their students work rather than waiting for a masterpiece to arise.