I don't know what your school is like, but when I was NHS Advisor, we didn't have enough team captains or club presidents to make up an NHS chapter. So those were not the only kinds of leadership positions that we looked for.
The NHS guidelines say this about leadership:
Student leaders are those who are resourceful, good problem solvers, promoters of school activities, idea-contributors, dependable, and persons who exemplify positive attitudes about life. Leadership experiences can be drawn from school or community activities while working with or for others.
If you look at those criteria, you can see a lot of stuff that doesn't involve having a title. I would use this list and try to pick examples of things that I do at school, at home, on my sports teams, in my clubs, whatever, that fit these criteria. You know, stuff that shows your dependability, your positive attitude, things like that.