Not sure if I have so much to offer Steph, but off the top of my head ...touching on what a story comprises, beginning, middle, end.

I'd steer clear of the 'complications' that are often added. in fiction shorts. I think that complicates things too much. That can happen down the tract if you're asked to do another one.

Talk about writing the story first with regard to paucity of words, but not to an actual 100 words initially.

After that's done they can do the weeding to 100 words which can sometimes be as easy as making some contractions ... he had, to he'd, etc.

Also you might have them look at padding words that novice writers tend to use. If they read allowed they will hear them.

Have you thought about modelling a drabble? I never ever asked my students to write without showing them how the process works. That can be a situation where you give them a title and they have to offer a first sentence, and someone else offers the second, etc. (I used to talk about buying their ideas, or not, but telling them they could keep it for their stories, so they don't feel put down).

You could have a title, and put your audience into small groups and ask them to construct a drabble in a given amount of time.

Hope this helps.