If your girlfriend enters Canada legally and then the 2 of you marry, you can then start the application process for her Permanent Resident's status. This process will likely take at least 12 months to complete and perhaps longer. Until she has PR status, she will not be able to legally work, attend school, or be eligible for government health insurance.
Being married to a Canadian citizen does not permit another nationality to pursue work or school opportunities.
To find out if it's possible to apply for a student visa or temporary work permit while awaiting the disposition of her PR application, why not call the folks who really know all the answers?
There is a toll-free phone number on the Contact Us page of the
Citizenship and Immigration Canada web site. The CIC advisors are generally very knowledgable and helpful. Yuo'll also be able to explain your exact circumstances and receive official answers.