Look into getting into Boys State. It is a program during the summer, between junior and senior year, for about a week that helps in the leadership category.
I completely agree with Boys State. My DS already had a big background in leadership, getting into BS took him even further. He learned so much and he loved every minute of his experience there. At least once a week he STILL talks about everything he gained in that short week. In fact, he even gets to go back again this summer as he was elected Lieutenant Governor for our state (though he wishes now he had run for president, lol!)
Remember, too, there are many paths to leadership that may not always stand out. Athletes will soon be training for Special Olympics Summer Games- get involved in coaching (believe me, the rewards are immeasurable, your heart will grow two sizes.) Give a talk to a local FFA or 4H group about something you know about and love. Do you know how to cook or do other Home Economics type things? Offer to lead a class for other young people as HomeEc isn't much of the curriculum in many schools these days (if you don't' already- one of the best ways to learn something is to know you have to teach about it- you'll learn really well if you know you have to teach it.) In band or orchestra? Figure out how to lead an ensemble (trio, quartet, quintet) for a demonstration. Initiate something!
One of the best and biggest things you can do in implementing your own leadership opportunities is show your initiating things. Leaders are starters, they are not sit-on-your-hands-and-waiters.
My DS says: DO NOT BE AFRAID TO STEP OUT OF YOUR COMFORT ZONE! Go out there, put yourself out there, be a dork, dance in front of the student body, help make your school's talent show something bigger, better and more!
Finally, do things that are in your heart- something that is important to you. It will show through if you are just trying to pad your resume. Along those same lines, though, be sure you TRY lots of new things- you don't know if you are good at something unless you give it a whack!
Aim high, kiddo- even if you miss, you will land among the stars! You can do this, I believe in you!