What does Milsim have to do with educating kids? Make them join the boy/girl scout, cadets, stuff like that..... What happened to the good old camping trip? Making the kids setup their own tent, collect their own wood, start their own fire, cook their own food, make them go on 1-2hrs hike, and make them clean it all up after.
If you want them to experience the life of a soldier, ie fear, split them in 2 groups, make them dig trenches, then make them wait in it for an hr or two, then cover up the trenches. Then make them dig it again 3 ft away from the old trenchies, and make them wait again. Give them the impression the other team is about to attack. Weapons? Give them waterguns. Giving kids under 18 each an airsoft is just plain dangerous since 1-2 teachers simply isn't enough to really safely supervize them