Soukoku was sent on a mission to get information on whoever, the easiest way to do that with the lowest suspicion was to join a school nearby. Dazai went by the name Shuji Tsushima as a cover up and Chuuya went by Chuuya Verlaine (since he couldn't come up with much). They had both already become executives, Chuuya just becoming one about a week before the mission started.

Dazai opened his locker seeing a note, as he opened it Chuuya leaned on the locker next to his.

Chuuya: "Whats it say?"

Shuji (Dazai): "I'm reading it."

'Meet me on the rooftop after 5th period'...

Chuuya: "It's probably just some shitty love letter."

Dazai stared at it longer, knowing something was off. Though Chuuya ignored him... which as usual Dazai ended up right in the end.

When he met you on the roof, you indeed plan on confessing to the boy, but also had a second confession to make.