I have never taught them leadership skills nor have seen a class for them. I know there are high school electives in that are for student leadership.
The thoughts I am having about leadership would involve having them group up into a couple teams and research local charities to sponsor. The groups would then create a presentation to the class and the class would then decide which one to sponsor (or the groups could simply sponsor their choosen charity and present the results). The leadership development would come from working collaboratively in a variety of roles.
I have seen this project done in an ELA 7 class, and although it is not a leadership lesson it definitely integrates the skills.
Leadership skills can be part of just about any lesson where learners are given the opportunity to make a choice. Whether it's a choice of what they are going to do (content), how they are going to work together (process), or what kind of product they are to produce (product), the learner should be empowered to take part in the learning process as much as possible. Resolving conflict, reaching a consensus, and assuring that every member has a voice all contribute to leadership values that promote tolerance, respect, and dedication to name a few. The maturity level of the learner is an important factor when designing a lesson plan that involves leadership skills, but the teacher's role should be, in my opinion, one that attempts to facilitate leadership skills on a day-to-day basis.
When evaluating leadership skills, assessing the process becomes equally important (if not more important) than assessing the product. Careful consideration should be made as well if standards and specific behavioral objectives drive what students should "learn". Since students each learn in different ways and at different times, expressive objectives thus become more suitable when assessing content, process, and product.
21 Keys is a program out of the Pacific Institute. Peoria schools have done it and I have an instructor who now works for me and she does some things with the students. It is powerful and the kids love it. It is also VERY expensive.