You might find something useful here:
Primary Resources: English: Text Level: General Ficition (Story) Writing
Could you make a checklist together of what features a story needs to have and while/after they write they decide whether they included all the criteria e.g. in your case "Sentences should be short and clear". Maybe you could also include features like "Include at least two time connectives" or whatever seems suitable.
After that you could get them to evaluate two examples of written stories that you've written yourself to ensure they've taken everything on board.
Hope this is a bit helpful!