The Enlistment — Janet R. Stafford

It is August of 1862 and sixteen-year-old Frances "Frankie" Blaine learns that her beau, Patrick, will be enlisting in the Army. In the flush of first love, Frankie wants to be by his side. She also wonders why she can't enlist and fight in the war too. Since no one seems able to give her a rational explanation other than that she can't because she is a girl, Frankie comes up with a plan. But it backfires and thrusts her into a world that she didn't expect and from which she learns some large lessons.