Look into civil air patrol/sea cadet units if your school does not offer a JROTC unit. Though if you're in your junior year or senior, I wouldn't bother with that. Eagle Scouts is also a good route. Organize a community service event, or fundraise out of school for a local natural disaster. Church groups are great for getting something like that started. Another thing I do is assist the church management wherever I can. Whether its putting a slide show or installing wifi in the church. It is difficult to find out of school leadership opportunities, so most of the time you'll have to make them yourself.
What year are you in school ? Why aren't there opportunities at school ? You can't just show up at school activities and "become the leader." It usually takes time, active participation and evolving into the leadership positions. That is why its important for candidates to start preparing for their USNA application early in the HS career.
If there truly aren't opportunities at your school...the advice about creating your own is good.
^^ Based on their prior posts, person would now be a junior in h.s.
I agree that becoming a leader in a particular sports team requires dedication and demonstrated performance. You don't just join a club/ECA and suddenly become the 'leader' and other students are usually wise to those who want to be he leader simply to pad their resume. Hard to imagine any h.s. that has sports/ECA's, etc. that would not provide leadership opportunities.
We have a charitable organization in our town that has a clothes closet, similar to a small, local version of Salvation Army /Goodwill. This entity also has a food pantry. The organization has a student executive board as well to incorporate youth leader involvement. My DS volunteered at the clothes closet 4 hrs each Saturday morning for 2 yrs, pricing and folding clothes, sweeping the floor, etc. He also got appointed as the rep from his high school to the youth executive board. Just a thought, to check if there might be something similar in your area. My DS also went to VFW Boys State, as already suggested.