You have two options, as I see it.
1) If you want to talk to her, 'What did you do at the weekend?' is good between Monday and Tuesday, and then from about Wednesday onwards you can get away with 'Are you doing anything nice this weekend?'. It tends to make people talk a bit, but be prepared for them to ask you back. Think of something to say that preferably isn't 'Oh, you know... stuff.'
2) If you don't really want to talk to her, make an excuse. 'Sorry, I have to go for my [insert instrument of your choice] lesson', or 'Oh no, I said I'd meet with [insert teacher/student] about some homework!' are both good ones. And, in the event you can't/don't want to lie, you can tell her straight you don't want to talk, subtly insult her until she notices, or just RUN AWAY. I have, maybe twice, gone with that option, and it might not be most 'helpful' strictly speaking, but it WORKS. <-- It's perfectly acceptable for this to be your current face.
Good luck.
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'I may not amount to much, but at least I am unique.' ~ Jean-Jacques Rousseau
'I sometimes go to my own little world, but that's okay, they know me there.' ~ Joel Hodgson