She's a storyline character. So you have to activate her storyline. There's two ways to go about it.
1. Make start to insult some people your not attracted to around 3 to 4. You need them to dislike you. That will lead to one of them eventually stealing your clothes while you shower in the morning. Your BF will see you naked and confused. If you accept her help she'll give you some clothes. After which she'll say you owe her a favor. After she reclaims the favor hang out with her in the evening, and talk about sex. If you don't want to piss of random npc's you can try to worsen your relationship with the Bully and The Crush's Crush. Who naturally don't like you because of story. And if they live in your dorm they'll steal your clothes.
2. If you don't want to go the favor route. You can try solving the mystery of the Party Milkshakes. Which you can do in one of two ways. Build up your Party popularity, or drink the milkshakes and start sucking dick. Eventually you'll either become popular enough that someone will let something slip, or you're not a quitter and you'll recognize a familiar taste in the milkshakes. Both will give you the prompt to investigate the milkshakes. After solving the mystery hangout with your BF in the evening and talk about sex.