There are some requirements you need to meet. You need all midnights events in the progress quests, you need to talk to Julia about virginity, you need to go to the school corridor to trigger talk with Oscar and tell Zoe you had a chat with him. Then you`ll be able to take the pill after 23:00 when going to sleep.I'm stuck at the "Special Chemistry". It's say you could use the pill at night but how?