You certainly can't assume you won't be charged, but I think the Air Canada employee would need to be having a pretty bad day to charge you $120 for a few inches! I am a student living in the UK and I have overweight bags quite often when traveling to Canada; however, due to the generous nature of the Air Canada employees (even when I didn't have status), I only pay overweight fees around 25% of the time. My advice is to be polite and courteous and the Air Canada employee may very well not even mention the size.

Or maybe the AC employees are trained to be more lenient with students... ^