Village Education Project – www.villageeducation.org
The Village Education Project is working in Otovalo, Ecuador. Volunteers work at a local school and live together at a hostel in the center of town. The program seems to mostly attract college students as volunteers, but they apparently will accept mature high school students. The program cost significantly less than most international service programs, but I also think this reflects that the volunteers are less tightly supervised.
Athenian Community Service Blog 