Pick a win-able local battle (even one that is all but won already) and build from there. Take a position on an issue, however small and local it may be, and get a taste of what making a difference feels like. Young people tend to be less patient and that actually is good and bad. The good part is that you're motivated to see some changes soon; the bad part is, do you get discouraged when it doesn't happen quickly enough?
My other thought on this is to check for this book in your school library or get a copy from Amazon and pass it around: "Free the Children" by Craig Kielberger. It tells the story of one kid who changed life for the better for thousands of kids around the world. The early chapters talk about what happened in his school as he began organizing. An incredible and absolutely true story. Craig is in my opinion, likely to be the Prime Minister of Canada at some point in the future. More at:
http://myhero.com/myhero/hero.asp?hero=c_Kielburger