During the First World War, very real fears about the food supply led to a whole host of changes to American agriculture, including farm labor. Although farmers lobbied Congress and President Wilson to exempt farmers and agricultural laborers from the draft, they were not exempted. So when the Selective Services Act was passed on May 18, 1917, farmers worried about who was going to harvest their crops. The wages of experienced farm hands began to skyrocket, and state and local governments scrambled to find a solution while the Federal government remained relatively hamstrung by the hold up of the Lever Act which would fund the U.S. Food Administration (and which would not be passed until August, 1917). Indeed, the U.S. Department of Labor would step in to help fill the gap in finding workers for many wartime labor needs.
For American suffragists, the Woman's Land Army was a reasonable solution. But rural and agricultural folks are fairly conservative, and the idea of young city women in overalls (scandal!) working their fields was unpalatable for most. Teenaged boys, however, were a good solution, in the eyes of many. Not only were they out or nearly out of school during key planting and harvest times, organized labor camps would train them for military service. And indeed, that's how many of the farm cadet camps were organized - with military tents, uniforms, ranked officers, and military language.
Gary E. Moore has written a nice overview of the U.S. Boys' Working Reserve, which was a US Department of Labor program started in June, 1917.
You can also read a 1918 report from the Bureau of Educational Experiments (no really, that's what it's called), entitled "Camp Liberty: A Farm Cadet Experiment."
The Farm Cadet program (like the Woman's Land Army and victory gardens) was revived for service during the Second World War. In 1947, following the war, the United States Department of Agriculture published, "Farm Work for City Youth," a glossy, photo-laden pitch for the value of agricultural labor, rebranded as "Victory Farm Volunteers."
The need for seasonal agricultural labor today is filled largely by migrant workers, many of whom work in appalling conditions and for poor wages. There have been improvements in recent years as various states implement minimum wage requirements for agricultural workers and mandate things like breaks, restroom facilities, and on-site water. But I wonder, if teenagers (especially white, middle class teenagers) continued to work in agricultural labor on their summers "off" - what would our agricultural labor landscape look like today?