I've honestly never given any thought as to how an American high school student would enter a Canadian university after high school to do an undergraduate/Bachelors program, although I'm sure it's fairly common. My hunch is that it is entirely possible, however you should really check out the admissions process yourself. I believe the web-site at the University of Toronto is www.utoronto.ca
While getting an undergraduate degree is one thing, I should warn you that entry into a Canadian *medical* school is difficult, if not impossible if you don't have Canadian citizenship. I only know of McGill university in Quebec, and Memorial university in Newfoundland who accept American students. I would check out Sean Winters' site. It's listed as the first entry under my links page, and has links to all 16 Canadian medical schools. For UBC, my school, if you were to attend UBC for undergraduate studies (ie. the four years Bachelors program done after high school), you would satisfy the BC residency requirement to apply to UBC. If you managed to obtain a Canadian citizenship in that time, there would be no obstacles that I know of, to you attending UBC medical school.