score:5
I've never been in the exact situation you describe, but Visa Waivers are a very low-maintenence way to get in. My advice would be to explain to the immigration officer when you go into the US. I'm pretty sure the answer will be that they will give you a new I94W on the spot (with of course a new fee, if they still do that).
You backup plan would be that if they so "no you can't do that", enter the US on your current I94, then exit at a convenient place before it expires, hand over your I94 to confirm that you have exited the country, and then re-enter it immediately (maybe a day later to allow for it to process through the system) and apply for a new I94.
Upvote:0
You may well have a problem at the border, but should be able to resolve that with an explanation of what happened. Do be sure to bring proof of your residence in Canada, studying etc. Also bring evidence that you really spent the last couple of weeks or months in Canada.
The big concern here is that at first glance it looks as if you were trying to game the system and stay more than the three months. You studying in Canada addresses that concern.
Generally, the rule of thumb is that you are only supposed to use the visa waiver once per year, but this is not written in stone.
You are probably better off waiting with your return to the USA until after the old I-94W has expired.