she'd have to hit the edge of his glasses and knock 'em off, but I think it could be done.
I certainly agree that someone could knock a pair of glasses off by hitting them just so...my two year old son recently did it to me (active little kid!). It's just that I noticed when it happened. In your story, Clark doesn't seem to notice that it happened. Well, Clark is very neurotic about his glasses - always seems to be aware of whether they're on or off. It would just ring truer to me if Lois slaps the glasses off and Clark does nothing to retrieve them, silently acknowledging that it's well past time that he reveal his secret to her.
Of course, it's a minor point and has not hindered my enjoyment of the story.

Susan