The difference between U.S. immigration status and a visa stamp is an important and sometimes confusing concept. F-1 status can be extended, shortened or even changed to a different immigration status (such as the H-1B) while you are inside the U.S., yet your F-1 visa stamp does not have to match your immigration status while you are inside the United States.
The visa stamp is only necessary for travel purposes. Initial entry and subsequent re-entry into the U.S. from abroad requires a valid visa corresponding to your particular immigration status at all times. However, you do not need to depart the U.S. if your visa stamp expires as long as your F-1 (or other) immigration status is still valid.
The U.S. Department of State has more information about the visa application process, visa fee payments, and reasons for possible delays in the issuing of U.S. visas.
Canadian citizens are not required to obtain a visa stamp from a U.S. consulate. Canadians are inspected at the U.S. border and admitted there in F-1 status