I like your idea of having college students teach high-schoolers in their communities about nutrition and physical activity because it makes good use of already established community bonds. I think that one of the difficulties that researchers run into when they try and make changes within a community is that they may not understand all of the dynamics and unique characteristics that make up that community. Your intervention could allow input from community members to tailor the message to their community, (hopefully) making the intervention more effective and giving it a broader reach. It also allows the community to take ownership of its success, which I imagine goes a long way towards the changes lasting over time.