The Student Conservation Association (SCA) offers qualified applicants the chance to lead volunteer crews in more than 100 spectacular national parks, forests, refuges, and urban sites across the United States. Crew Leaders work with crews of 6 or 8 high school students, in backcountry or front country sites.
http://www.thesca.org/green-jobs/sca-national-crew-leader-opportunities-summer-2010/various