Ed's mum wrote:
Good and varied punctuation.
Reasonable spelling, or plausible alternatives.
Interesting and engaging to read.
Varied vocabulary - adjectives, adverbs, 'wow' words (more expressive/complicated language).
A variety of styles of sentence: Complex sentences, short ones for impact, variation in the way sentences are started (starting with an adverb for example).
Consistency in the tense used (unless the story flits between times!)
A beginning, middle and end.
Don't have too many characters in a short story. Two at the most.
Perhaps a description of the setting.
Maybe some speech, but not much or it becomes a playscript.
I like these ideas as I am working with my son, and have used much of what has been mentioned above. There has been a marked improvement in his writing in just two weeks.
Some little tips to enhance this.
Always use alternatives to "and" & "but".
Always have simple question & answer dialogue.
Avoid repetition (e.g Jack & Jill can be the siblings, the children, the pair, the twosome etc.).
I would also recommend getting an A4 sheet with all the things they need to do and to have this as aide-mémoire in front of them while they write. Obviously they won't have it in the exam, but by then they will have pretty much commited most of it to memory.
The time given in these tests means it is best to have a basic formula and show off your writing as effectively as possible. There isn't the time to be creative.