The Water War by Kevin Miller
When a beautiful but mysterious new girl moves to the small prairie town of Milligan Creek, everyone is so smitten that her mere presence threatens to ruin the perfect "boys only" summer that Matt has planned for him, his brother, Chad, and their best friends, Andrew and Dean. So, the boys come up with what seems like the perfect distraction: a water war. It’s the ultimate game of survival, where every player is both hunter and prey. But when Matt's buddies invite the new girl to join in, the game that was supposed to unite them against the interloper threatens to tear their close friendship apart.