To trigger Still Young you need to have done Sana's "Shaking the Tree" event, Ayane's "Endless Torment" and "The Worst Thing Ever" events, and then visit Ayane's dorm room on Saturday.
To trigger Under the Table you need to have done her "A Book About Dragons" event, the main event "See you in the Morning," then visit the library.
To trigger Worse Comes to Worst you need to do Rin's "Nothing was Missing, Except Me" event, have 15 Affection with Yumi, and visit her dorm room.
It sounds like you're stuck on the Reset main event, at which point just visit the shrine until it says your affection with GOD has increased to 5 or more, then go to school on a weekday.
The big things to avoid missing scenes before going into the beach event are don't do Chika's "Schadenfreude" event (you can do it after the beach event, just not before), and have Ami and Ayane over 10 lust. There are a few other things you can do to change the scenes slightly, but I can't remember them exactly, and this will atleast keep you from missing any scenes. If you want super specific info about the beach event, somewhere around page 120 on the forum there's a huge discussion about it and all its variants.