An excerpt from UWaterloo website:
Canada and the U.S. have a special agreement, (Regulation 190 3f), which allows holders of a valid Study Permit to re-enter Canada after a trip solely to the continental U.S. It is strongly recommended, however, that you apply for the Temporary Residence Vise (TRV) especially if you are travelling by air. Airlines may not allow you to board the flight if you do not have a valid TRV.
Print out 190 3(f) and bring it with you:
http://laws-lois.justice.gc.ca/eng/regulations/SOR-2002-227/page-79.html#docCont
(f) to re-enter Canada following a visit solely to the United States or St. Pierre and Miquelon, if they
(i) held a study permit or a work permit that was issued before they left Canada on such a visit or were authorized to enter and remain in Canada as a temporary resident, and
(ii) return to Canada by the end of the period initially authorized for their stay or any extension to it;
I've done this myself, and by showing my work permit I wasn't given any trouble boarding the plane. I highly recommend you to bring a print-out of 190 3(f) just in case someone gives you a hard time.