The simple answer to your question is that gray had long been the color of uniforms worn by many pre-war American militia units and cadet corps. In addition, some American soldiers had worn gray fatigue uniforms in the War of 1812. Since the Regular Army of the United States wore blue uniforms, the Confederates selected the next most popular and traditional color of uniform. Of course, during the early stages of the war, a number of militia units from the North still wore gray uniforms while some from the South wore blue. This led to considerable confusion during several battles, like First Bull Run (Manassas).
I hope this information helps in your school discussion.
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