Day Programs: French-American Schools within the US often have summer programs through Middle School. Here are links to 2 schools: www.fasps.org and sdfrenchschool.org
Away Programs: http://www.canoeisland.org/
This one goes through high school I believe
My children have been to both the French American school camps I listed, and they are a great, immersive experience. I have heard really great things about the Canoe Island camps from friends.