Everybody here quoting 3 weeks max but no one posts a link to any official guidance. People who get refused entry are generally trying to come in a few months before and less than convincing they can support themselves without working before classes start which is the main concern of CBSA.
If you arrive Dec 18 you then have to quarantine for 14 days which takes you to begin January leaving you very little time to sort out anything at all. So being 2 days over 3 weeks will be the least of your problems given big part of your quarantine is over the holidays and you are not allowed to leave quarantine at all unless a medical emergency.
CBSA of course have the final word but if you have an isolation plan and any other documentation to support your study permit and college being on the COVID ready list personally think you may be over thinking things.