I'm drawing all of my knowledge from my experiences as a student in Vancouver, BC (Langara College). There may be slight differences in the off-campus work permit program in other locales, but from my understanding its administered pretty uniformly Canada-wide.
There are essentially two ways in which an international student can work in Canada ...
1. The first category involves on-campus work only. As these jobs are administered by the schools themselves, no work permit is required. The schools set their own rules regarding eligibility requirements for these positions. Although a work permit is not required for these types of jobs, a SIN is required (for purposes of taxation), which can be obtained at any Service Canada location.
2. The Off-Campus Work Permit is an altogether different animal!
The Off-Campus Work Permit Program is a collaborative program. The program was established by CIC, but schools play a huge role in it's implemenmtaion. A post-secondary institution must participate in the program in order for its students to be able to apply for work permits. There are significant hassles for the school(s) involved and a considerable workload and expense (schools usually must have several full-time employees dedicated exclusively to supporting the program), so many schools simply elect not to participate.
Basic requirements. Unless noted, these are uniform and do not vary from school to school.:
1) Student must have a valid study permit.
2) Student must be registered as a full-time student in an academic program, carrying a minimum of 9 or more credits during the fall and spring academic terms. During the summer term, students may take a lesser course load (or none at all) without negatively impacting their work permit.
3) English or French as a second language courses cannot be counted towards the minimum 9 credit hour requirement.
4) Before a student can submit an initial application under the program, he must have been registered as a full time student for a minimum of 6 months. In order to maintain eligibility, student must always have been registered as a full time student for 6 out of the previous 12 mounts.
5) Students must achieve a minimum GPA each term. At Langara College, the required GPA is 2.5, other schools may have lower or higher requirements. In no case can the required GPA be lower than 2.0. Note that this requirement mus be met each term. If a student ever has a term during which his GPA is less than the required minimum, even if his overall GPA is much higher, then that term doesn't count towards the six-month minimum. It is possible to have one 'BAD' term (but no more) in a year, but only if you make it up by taking a full courseload during the summer term (and achieve the GPA minmum during that term. Otherwise, it is impossible to meet the 6 out of 12 month requirement.
Assuming the best-possble scenario, a student can apply for an off-campus open work permit no sooner than midway though their second term. The process works something like this ...
1) The student must attend a mandatory, one-time informational session that outlines the program. This session is presented by the school but is overseen and has been approved by CIC. At my school. these info sessions were available once a month, other schhols may do things differently.
2) The initial application MUST be submitted online. When CIC receives the application, they contact the schhol for verification.
3) The school verifies eligibility requirements have been met (namely full-time registration status and satisfactory GPA for each term). If 2 full terms haven't yet been completed by the student, i.e., if he applies mid-eway through his second 4-month term, the school verifies with instructiors that the student's current term average meets the minimum.
4) Student receives an email from CIC saying that they have either met the requirements or been denied a permit.
5) It the sudent has not already done so, payment of $150 must be made.
6) Once the work permit has been issued, student goes to nearest Service Canada location for issuance of a temporary SIN. Service Canada will issue the SIN to the student immediately, and the actual SIN card will follow by Canada Post within a few weeks.
Once you have the SIN, you can legally work off campus.
Important Notes, Restrictions, and Useful info ...
1) remember, that expiration dates are always based on expiration of primary status documents. The primary status document for international students is the Study Permit. Off-Campus work permits, temporary SIN, etc. always have expiry dates to coincide with the expiry of the Study Permit. Work permits cost $150 regardless of the length of time they are issued fior. If your study permit is close to expiring and you will be renewing it soon anyway, then wait until you have done so before applying for the work permit, as you will have to renew that as well (for another $150) once you've done so.
2) There are restrictions on the amount of hours that can be worked. You are entitled to work a maximum of 20 hours per week during the regular academic year. During scheduled breaks, you may work full-time. Scheduled breaks include summer, winter holidays, spring break, and the short break following final exams prior to the staqrt of the next semester.
3) Each term, the school is required to submit to CIC a list of all students who have become academically ineligible for the Off-Campus work permit, either due to not meeting the minimum GPA, or because they failed to maintain the minimm full-time requirement of 9 credit hours. In my experience, schools are extremely dilligent in meeting this CIC-madated requirement. Not doing so would jeopardize the school's inclusion in the program and potentially penalize all international students participating in the program. As a result, schools don't cut students much slack. If you fail to meet the requirements, you WILL be reported as being inelligible.
***IMPORTANT***
If at any time you no longer meet the requirements for the permit, you are required to surrender the work permit immediately. Simply stopping working is not sufficient. In the past, students, upon becoming inelligible, have simply stopped working and kept the permit, thinking that once they improved their academic standing they would become elligible once again, and then staqrt working again. THIS IS INSUFFICIENT. If at ANY time you become inelligible, you must surrender the document. Once (If) you become elligible again, you must reapply for another work permit.
I CANNOT OVERSTRESS THE IMPORTANCE OF ADHERING TO THIS POLICY. For any international students (and I know there are many of them) who intend/aspire to become permanent residents following graduation by obtaining a PGWP and later applying under CEC, violating these policies can have a profound and lasting negative impact on your long-term goals and intentions. If you become work-inelligible for any reasion and yet continue to work, you will have broken immigration law. As the saying goes, there is no way you can "un-ring" that bell. Just dont't do it!
I realize that my comments may be somewhat disorganized, but I hope that they will serve to answer some of your questions.