Him rubbing his palms together signifies many things...umm, I wouldn't reveal them. He had something else to say to them regarding Monk Talon's scroll and embarking on the mission, it was of great importance.
Remember, it could be that the F class students managed to get to Monk Talon's house, got the scroll, but died on their way back. It would be really painful for them to get to Monk Talon's house and not see the scroll where it was supposed to be...it would be wasted effort.
Captain Wayne wanted to tell them something regarding that, but he decided that he'll tell them on the day they are to embark on the mission or close to the day. Since he's approved of it, then they'll go on the mission, no one can disapprove of it. He's the only one that can decide on things like this since he was the captain.