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No way to give advice without getting into semi spoiler territory, so spoilers if you read more:

If you're a guy you just need to keep attending and increase your party popularity, and a eventually you'll have a suspicious conversation with one of the guys at the party.

If you're a girl you'll need to get some sexual experience (just oral, and be sure to swallow). Practice your oral skill and keep accepting milkshakes that get handed to you and you'll notice something off about the flavor.

Once your character has suspicions about the milkshakes just hang out inside the dorm at the party. Keep getting drinks or snacks and eventually there will be a weird noise and an option to investigate.