Some of my students do volunteer work at Wildcare, a facility in San Rafael, California that provides veterinary care for sick or injured wild animals and lets the rehabilitated ones go back to the wild. They help care for sick wild animals or help run educational programs for younger children and visitors. http://www.wildcarebayarea.org/volunteer-opportunities/volunteer-young-adult/. Similar wildlife rehabilitation organizations in other parts of the country that accept teen volunteers (because not all of them do) include:
The New England Wildlife Center (Boston Area) http://wildlife-education-center.com/
The Wild Bird Fund (New York City) http://www.wildbirdfund.org/
Tri-State Bird Rescue & Research (Delaware) https://tristatebird.org/
The South Florida Wildlife Center http://www.humanesociety.org/animal_community/shelters/wildlife_care_center/
The Sarvey Wildlife Care Center (Seattle Area) http://www.sarveywildlife.org/
Some of my students help care for sick seals getting rehabilitated at the Marine Mammal Center in Sausalito, California. They prepare meals, clean pens, interact with visitors, and sometimes feed the seals. It’s fishy, smelly work. http://www.marinemammalcenter.org/Get-Involved/volunteer/youth-crew/ There are other such facilities around the USA, but most do not accept volunteers under 18 years old.
The Bronx Zoo (New York City) has a program called Project TRUE (Teens Researching Urban Ecology.) You work in a team on a research project about wild birds or invertebrates in the city. You can do it during the academic year, or during the summer. The summer program pays a stipend and gives you a MetroCard. http://bronxzoo.com/teens/project-true Also, if you are sixteen you can volunteer to be a summer Discovery Guide at the Bronx Zoo. http://bronxzoo.com/volunteering/discovery-guide-volunteer-program
At the Smithsonian National Zoo (Washington, DC) teens can volunteer to be Safari Day Camp Class Aides during the summer. You do not get to handle animals in this program. https://nationalzoo.si.edu/Support/Volunteer/Teens/default.cfm
The Brookfield Zoo (Chicago area) has a training program for high school students called King Conservation Science Scholars. After a year in the program you are eligible to apply for zoo-specific college scholarships and paid positions at the zoo. http://www.czs.org/Chicago-Zoological-Society/Conservation-Leadership/Conservation-Science-Scholars
Teenagers can volunteer at the Woodland Park Zoo (Seattle) through the ZooCorps Teen Volunteer Program http://www.zoo.org/sslpage.aspx?pid=1789#.VdX6eflVhHx
The San Francisco Zoo has many opportunities for youth volunteers as young as 11. Teens during the summer handle animals and interact with the public through the Nature Trail, Talk on the Wild Side, and Zoo Teacher Assistant programs. During the school year you can join the San Francisco Zoo Crew to restore wildlife habitat. All these programs are unpaid, but you get to work very closely with the animals. http://www.sfzoo.org/learn/volunteer.htm
The San Diego Zoo offers two programs for teens. In the Zoo InternQuest Program, high school juniors and seniors who have a strong interest in careers in the life sciences receive after-school training in conservation biology three days a week for seven weeks. The program is free. As a Zoo Corps volunteer you interact with visitors in the zoo. http://zoo.sandiegozoo.org/content/teen-programs
The New England Aquarium (Boston) offers paid and volunteer teen internships, a service learning program and a Marine Biologist in Training Program. http://www.neaq.org/education_and_activities/programs_and_classes/teen_programs/index.php
The Woods Hole Science Aquarium (Woods Hole, Massachusetts) has six week unpaid summer internships in marine biology for high school students. http://aquarium.nefsc.noaa.gov/hsinterns.html
The National Aquarium (Baltimore) has a volunteer Student Summer Program for high school students who have taken a year of biology. http://www.aqua.org/learn/student-programs/student-summer-program
Teens 16 and older can volunteer at the Georgia Aquarium in Atlanta. http://www.georgiaaquarium.org/experience/support/get-involved/volunteer
Chicago’s Shedd Aquarium has about six programs for teens 15-18, including a paid work-study experience: you can become a Shedd Ambassador or a Youth Naturalist. http://www.sheddaquarium.org/Learning-Experiences/Teen-Programs/
The Seattle Aquarium has a High School Volunteer Program, a Youth Ocean Advocates Program, and a Creatively Inspiring Conservation (Art) Program. www.seattleaquarium.org/
Teens 14 and older can volunteer at the Monterey Bay Aquarium (Northern California). http://www.montereybayaquarium.org/education/teen-programs
The American Fisheries Society Hutton Scholars Program places high school juniors and seniors across the USA in eight-week, full time summer work with a fisheries biologist mentor, within commuting distance of the student’s homes. They earn a $3000 stipend. http://hutton.fisheries.org/information-for-students/