I'm wondering how far in advance I can come to Canada from America on a student visa before my school starts.
You can travel to Canada as soon as you receive your letter of introduction and your passport is returned to you from the consulate. It will already contain a valid visa if your nationality requires a visa to travel to Canada; otherwise Canada will automatically have given you an eTA if your nationality requires one. You need to bring the letter of introduction, your passport and possibly other documents with you when you travel to Canada; your actual study permit will be issued to you at the border when you present your documents to the CBSA border officer.
The letter of introduction will specify the latest date which you can enter Canada; if you do not enter on or before that date, you will not be able to obtain your study permit or study in Canada.