They Were Eleven (1986) IMDb
The far future: Terra is at peace with several other interstellar powers, including many obscure human races. An academy welcomes them all, offering a series of harsh tests for a prestigious certificate. The last test is to survive totally isolated on a derelict ship with nine other students, but when our hero gets there, the strangers number ten. It’s a reverse murder mystery! Making contact with the outside world before 53 days are up means surrender and failure for everyone.
Medium-hard science fiction mystery. A nice elastic universe where anything goes but not too far. The mood is almost serious, with only one bizarre J-pop food fight. Features a consummate bishounen. Based on a manga.
References here: Gall Force: Eternal Story (1986).