There are lots of photos that show the awfulness of life in the trenches without showing the blood, guts, and dead bodies; but sometimes showing stretcher bearers and the walking wounded. Similar dioramas will be just as effective without the risk of offending people, or being accused of trivializing matters. Schools may also have reservations about some scenes.
There are scale vehicles from this period and there are also positive scenes, like the Xmas 1917 football game.
A diorama could also be part of something bigger, and augmented by storyboards containing pictures of shell shocked troops, quagmired vehicles and pack animals, etc. Remember that your audience (school kids) are part of the connected generation. Adding things that are interactive may increase their attention.