It should work any day after school once you've had the conversation with her in which the fight it set. I've just had a look and it's working fine for me, so feel free to email me the save and I'll see if I can figure out what's gone wrong.
I figured it out. I had to tell her to fuck off in order for the quest to progress. For whatever reason, just telling her you're gonna kick her ass doesn't progress the quest. At least it didn't in my game because I chose that option at least twice and it never progressed.
