I'd be more direct if I were you. You can't know why she didn't answer. If she's sick or her computer died you could end up feeling very bad or foolish? Not a good use of your anger. Rebelling is for kids and won't get the respect you want and deserve.

Go and start the sesson with a very blunt, "Why didn't you answer my e-mail like I asked you to?" And if you don't like the answer, you can give her heck or get up and leave? That would be lots more effective because you could see the response and be part of the whole thing.

Of course, starting out accusatory, like there's no good reason for someone not to do what you ask them to and like she's deliberately blowing you off could be embarrassing too if it turns out she has a good-enough reason?