You dont need work permit because TA and RA is 'work on campus'.
As in CIC , http://www.cic.gc.ca/english/study/work-oncampus.asp
You may work on campus at the institution where you study without a work permit if:
you are a full-time student at:
a public post-secondary institution, such as a college or university, or a collège d’enseignement général et professionnel (CEGEP) in Quebec or
a private post-secondary institution that operates under the same rules and regulations as a public institution, and receives at least 50 per cent of its financing for its overall operations from government grants (currently only private college-level educational institutions in Quebec qualify) or
a Canadian private institution authorized by provincial statute to confer degrees and
you have a valid study permit.
You must stop working on-campus on the day you no longer meet the above eligibility requirements (e.g., if you are no longer a full-time student.)
