Does the university give a word count for the contest? Though flash is often 1000 words or less, some places set a lower limit.
Structure it like a normal story, with a beginning, middle and end. It should have a plot. The only difference with flash is economy of words... you can't waste any.
The best way to get a handle on it is to read some. You might want to check out the '
polished and ready' subforum of this one, for stories being critiqued (password is flashed). There are also a few examples in the
March FAD entries thread (sticked in this forum), and in flash fiction magazines (there's an
index of magazines in the discussion sub-forum... a fair number of those listed under 'f' are flash fiction only, because flash fiction magazines like to start their name with flash).