11-nin Iru!

They Were 11 is about ten Galactic University recruits who have passed many levels only to be given one final test. They are told to travel to an ancient spaceship and live there, together, for fifty-three days. This is not a problem until the ten recruits, hailing from diverse backgrounds and planets, discover that there are actually eleven of them on the ship. In fact, this problem turns out to be the test, as they have to trust each other in order to survive, even though one of them is an impostor.

(Source: Viz Media)

Note: Won the 21st Shogakukan Manga Award for Shoujo in 1975.