Nazotoki Drill

Mikai Yui is a high school girl detective... sort of! She's already 19, but she's on her third repeat of the final year of high school. It seems like people get murdered around her all the time, but she's always excited to declare her expertise and trip over the body. Her "assistant," who she calls "Rinrin," is actually just a classmate (not an assistant or even a friend), who happens to be a lot better at figuring out murder mysteries. Their first murder of the series happens on a school excursion, when a maid finds a man stabbed to death in his hotel room!

(Source: MangaHelpers)