To open the safe you have to find the old HM's wallet. To do so you have to open lake access, at the Friday meeting, and then talk to the groundskeeper. He will eventually offer to show you the area where they found the old HM and that's where you get the wallet. Inside is the safe combo. Inside the safe is blackmail material to use against Ruth which is how you advance her story.
You should have discipline higher than 20 when u open the lake or else grades drop everyday. And I don't think you get the option for Jimmy to show you until after you have had the 3rd dream. The 3rd dream happens after you speak to Ruth about the old HM's locked journal entry on the comp. If you haven't gotten the locked entry unlocked yet it happens after Miss Chang shrinks Sally and they fight. Miss Chang loses hilariously but still gives you the password which unlocks the entry. Yes you have to speak to Jimmy on any weekday cuz on weekends he is passed out drunk. But once you have spoken to him about showing you the next weekday that you visit him he will take you.