The St. Louis Fed has published a concise and interesting short essay on farm subsidies. It provides some basic history, as well as a good explanation of the economics. Additionally, it provides some links to articles and data sites.
For a teacher of economics or political science, trying to explain how the program came to be and how it works, this is good basic source that many high school students should be able to understand. And if it is above your student's reading level, the article is short enough that you can summarize it during class discussion.
I look forward to your comments.