I have been involved with this once before, a few years ago now. It's a great experience for the kids.
I was lucky in that the teacher who was at my school before me had found and adapted a simple play from an online source. When I arrived all that was required was rehearsal time (and lots of it!).
The students were super dedicated to practicing during their lunch hour and after school and it paid off because they won!
We all attended the competition and I have to say, after sitting through hours of these plays, the best ones are the ones that are simple. Simple plot, simple dialogue, simple stage direction. If you manage to find anything online and adapt it, do it with that in mind.