High school graduation ceremony marks the end of one's youth. As the elder of the twins, Itsuki had "the best" ceremony surrounded by his friends and girlfriend. Alternatively the younger twin, Haruka, had "the worst" ceremony, having to spend the time on the roof alone.
During the ceremony, the two accidentally fell from the roof. To their surprise, they were both brought back to the entrance ceremony of their high school. From there, the two's "best" and "worst" youth had been replaced.