This manga already finished long time ago

i only remember the start and the ending but kinda forgot the middle story and want to re-read it again

The story was simple, a boy had childhood friend, another boy that looks so cute like a girl.

time skip

the boy in middle school i guess, meet with his long lost childhood friend but he dresses a girl sailor uniform. The boy then say " hey you are a boy why you are wearing that ? "

the chilldhod friend reply " you so silly, i was a girl ok"

in the end the "girl" become the boy neighbourhood, and the boy call the "girl" mother and ask her is his childhood friend a boy or a girl, and the mother say "oh him, he was quite difficult child but he's a boy"

But the boy doubt it, because the "girl" act and by some event "convince" him that she truly a girl.

So start the story about the boy who try to solve this "girl" gender, and the story is all about his effort, peeking, or do some weird thing to exposes the "girl" gender/

No Bra?