International volunteer opportunities for students (age 14 yrs to 22 yrs) are of two kinds. Personally paid volunteer travel programs or UN, other agencies paying for you. To get a free UN volunteering opportunity is not very likely because of the volume of the applicants and also time commitment (min 6 months or so). I’ll recommend GVI-Canada for a paid volunteer- travel program (IVHQ and Volunteer forever and many more links are below). These paid programs enhance students resume for future job prospects at UN, Intl. Development or local governments.
Volunteer forever is an excellent resource site for volunteer abroad programs. Pls see 925 programs they have on their website.
Volunteer Service Overseas. International program (Free).
UNV-United Nations Volunteer Program (Free).
Domestic-Canadian Volunteer Opportunities
Charity Village is the biggest Canadian volunteer resource site. Most non profits use their services for advertising volunteer opportunities.