If you really want to confuse the student
you could mention that the heavier aircraft actually does a little better than the lighter one.
All the (good) analysis so far assumes that the skin friction drag coefficient is independent of speed. It's not quite so, and it actually decreases slightly with speed (Reynolds number).
An aircraft that is 20% heavier, and therefore gliding 10% faster, has a skin friction drag coefficient between 2 and 5% lower, depending on the type of boundary layer. So it should get a better glide ratio, by a whisker...
That's one of the reasons why the big jets have such good glide ratios. They're big and fast, both of which raise Re.