I would imagine it is probably too late for you to use it at this point, but I know of a site that might work for you in future school years. This game is formatted like a "choose your own adventure" book. It gives the kids a variety of choices which direct the storyline. As with the actual trenches, most decisions lead to death, though. I have used it for two years with some of my classes, and the kids have fun and seem to get the point. I have them do a reflection activity where they place themselves in the trenches and have them think about the mental and physical toll that it would take on them.
Here is the website:
Hope this helps-