Mike_T said:
If I (as a US citizen) drive up to the Canadian border, present my passport and say; I'm coming to Canada to marry my fiancée and apply to a Canadian college, are they going to let me in, or turn me around?
Yikes. That story is likely to get you refused entry. How about "I'm here to check out X college and visit my gf." They may push you a bit to show you plan on leaving Canada, but they usually don't push US citizens all that hard - unless given something that makes them suspect you won't be leaving in 6 months time, like "I'm here to get married and apply to be a student." That's pretty much waving a pair of big red flags in front of them.
Don't lie, but don't make it out to be something they could interpret incorrectly either.
Make sure you also know how long you plan on staying (keep in mind - plans change. You might LIKE this college and apply on the spot. You'll still have to get a study permit, but that's pretty easy for a US citizen.) So when they say "how long do you plan on being here" say something definitive like "six weeks" don't say "well, that depends upon..."
The CBSA concern is the same as CBP - they don't want to allow in illegal aliens.
Good luck!