Talk to your teammates and eventually one mentions a blue haired boy watching you during practice, then go talk to him outside and he'll mention you never responded, this then unlocks being able to write a response in the library for him.
Clicking the trees does nothing unless you're at the part of the story where you're meant to go back there at night. If you can't find the right tree after clicking them all, then you need to keep playing through the game.
Explicit mode is enabled by clicking on the campfire outside Rai's old home in Miriko Village. The code is 2902.
It's just part of the story. Keep playing and you'll get to it.
