Ignoring the fact that he is obviously not a he, but a she... how can the guard say he's being silly; she's clearly trying to look like a male by binding her tits and wearing a male uniform. The misunderstanding she's laughing about was a purposeful thing on her (and the school's) end.
That said, it still doesn't make sense for him to have believed it. Why would they hire hot dude as another dude's guard, if men need guarding?... Why would they hire ANY men as guards for other men? And if the men need to have another man around them at all times for safety, how can he just leave he supposed male guard to walk home by himself?
Anyway, I'm liking the current "Chasity Reverse" trend. I'm sure I'll get tired of it when they release too many samey ones, like I did the "Villainess" genre, but I'm digging this for now.