Usotsuki Otona Change!

Synopsis

Futaba, a fifth-grader, has a crush on her childhood friend Takuto, who is three years older than her. But Takuto doesn't pay her any attention, saying she's like a little sister to him. Wanting to get closer to her, she borrows her sister's uniform and sneaks into her junior high school...!? (Source: Ciao Plus, translated)