He probably has this sense that even if they defeat the demon king they aren't gonna return to earth. Plus they'd catch on eventually that he's an assassin if any of them pay attention since his divine ability is "judgement" xD nowhere does that give you the ability to just skillfully assassinate something.
Oh sorry, I have the read comprehension of an ape. Fair that it's suspicious, though I feel like the students don't have the mental capacity to put two and two together from everything we've seen from them thus far. Hell if MC just said he unlocked a new ability I'm sure they would've ran with that since they don't really care/pay much attention to him. I stand by him having no reason to out himself, was just stupid and he's lucky who he told is equally as stupid to not even think twice about it

Why on Earth did he reveal the fact that he's from a family of assassin's? Like, sir you very obviously didn't have to do that? Man's the worst assassin I've ever seen, telling literally any mf his identity (she was nice to him for a total of 1 chapter, how does that make her trustworthy???).
I'm honestly not even surprised that the first person who he told was of course a girl, these manga seem to follow the exact same formula to a T, and as always everyone will be mad at the author for adding unnecessary harem or romance later. Big smh