Azusasou no Shokutaku

After completing their training with an assassin organization, four young people are brought in for their final test: to kill each other, to determine the sole survivor after six months. While you might think a brutal battle is about to begin, the four are enjoying their six-month reprieve, stretching their wings in the regular world! But of course, the assassin test continues...

(Source: Kodansha, translated)