So if you don't study hard, you won't go the same school as her. This will lead her to become somewhat distant, but if you were nice, it'll lead to a route where you still get together.
But if you choose decisions where you reject her (like saying you won't marry her and rejecting her Valentine's gift) and you choose not to study, she'll become so distant that she'll push you away and go out with someone else. You can either call out to her or ignore her. If you ignore her, it'll lead to an ending where you see her as an adult, but you don't recognize her.
If you choose to call out to her twice, eventually you two will go to her home after it is revealed that her boyfriend is abusing/beating her. You'll admit that you always loved her and basically force her to have sex.
It borders NTR because she still goes back to that abusive boyfriend, which leads to the ending where you meet adult her and not recognize her, and it borders rape cause, of course, you force her to have sex, but she wasn't completely against it since she still had feelings for you.
Still, if you want pure vanilla, not the route you want.