Uyamae!! Ryuunen Senpai

From MangaHelpers:

Ryuune Sakura is repeating her senior year of high school. Though she is older than any other student in the school, she is still so small and cute that people mistake her for an elementary school kid. She's determined to maintain respect (or maybe fear) from her classmates. Her plan is sometimes undermined by her classmate Mizuki, who keeps talking to her and who knows the real reason she's repeating the year. [tethysdust]