DUDE! - DO NOT F-ING DO IT. TRUST ME.
I got in big trouble a while back.
Four teen-agers is a red flag for them.
The people at the border have the right to do WHATEVER THE HELL THEY WANT! including shoot you - that's a fact.
The canadians can refuse to let you into their country for whatever reason they want - especially if anyone in your car has ever been arrested for ANYTHING, EVER. (the canadians probably won't search you, BUT...)
They will then send you BACK THROUGH THE US BORDER. If this happens, they will AUTOMATICALLY SEARCH YOUR CAR, and possibly your person (nutsack an all). They will PROBABLY HAVE A DRUG DOG. You will go to fucking jail.
I am not trying to scare you. This happened to me. I have been stopped a couple of times since and been hassled (not arrested again though, I didn't have anything). IT IS NOT WORTH IT.
Sure, 9 times out of 10 they let you right through, but it's not worth the risk, trust me. If you must, make sure you cover up the smell incase you run into sniffing dogs (like the peanut-butter idea.)
seriously, though, just chill. It'll be easier just to buy it up there.