Not every foreign student in Canada requires a study permit.
If you have a valid study permit you are allowed to work part-time on-campus at the university of college they are enrolled at if you are a full-time student/
If you can complete a course or program in the period of time allowed to you under your visitor status (normally 6 months) then you do not require a study permit.
If you are a minor child and your parents are in Canada legally and are not considered "visitors" then you can enrol in full-time education.
To be classed as a minor child you must be under the age set by the province you study in:
Alberta - 18
British Columbia - 19
Manitoba - 18
New Brunswick - 19
Newfoundland and Labrador - 19
Northwest Territories - 19
Nova Scotia - 19
Nunavut - 19
Ontario - 18
Prince Edward Island - 18
Quebec - 18
Saskatchewan - 18
Yukon Territory - 19
If you are a member of family or staff of a foreign representative or diplomat as accredited by Department of Foreign Affairs and International Trade then you do not require a study permit.