I think it's one shot and shoujo. The main girl likes this sempai and confess to him. He agrees to go out with her. It was found out later on that he agrees to go out with her because he was rejected by his crush and he think anyone is fine.

His crush likes him too and she said she rejected him because she wants to test him.
The main girl and the sempai ended up together.

This is confusing to read 😕

Ah sorry.. so here we go,

This main girl likes her sempai and she confess to him. He agrees to go out with her.
But later on, she realize that her sempai had a crush on someone else.

The day the main girl confessed to him, the sempai confessed to his crush and got rejected. The sempai agrees to go out with the main girl because he was heart broken.

Side note: the crush actually like the sempai back but she rejected him just to test him.