That is a performance defect with the Air Canada staff - it's on the company to provide adequate policies, training and enforcement to make sure the staff are delivering a uniform experience across the system so these issues stop happening.

Once Group 1 and 2 finish their boarding, that line should be kept clear and reserved for G1/2 customers boarding at their leisure, and the announcements should clearly state this - AND anyone using that line who isn't entitled, should be turned around and sent back to the end of the regular line. I watched one GA at LAX do this while boarding a flight and wished they could be cloned.

In a perfect world, they would staff one additional agent at the gate with a cattle prod to jab any G1 or G2 customers who think it's OK to push their way into the priority line from the side while it's boarding.