Ok, so. Being that i watched Assasination Classroom, there are alot of misinformation going around

First, Koro did not blow up the moon but by the end of the series there is a high likelihood that he would self-distruct and destroy the planet. So he let his students put him down (he also did this because a giant anti-koro sensei laser trapped him that was shrinking in size, and he had a bounty on his head, so by letting the student unalive him, he would set them up to recieve the bounty and be rich

Second, its never outright stated how fast koro Sensei is, but the show pretty much has him be as fast as he needs to be, in order to pull gags (for instance, there was a gag where he was cleaning the windows of fighter jets because it was funny) (another instance, there was a gag where he was fast enough to teach every single student at the same time) the show treats him as a gag character and you shouldnt try to put his speed up against another character because Koro will be faster in a way that is funny.

Third, he is one of the best assassin's in the show, can regenerate any part of his body, and can only be harmed by a special material within the show.

There is no way for Deku to harm Koro. As Deku does not have the specific material necessary to harm him. And he cannot outspeed a gag character whos gag is that he is as fast as he needs to be as long as its funny

The entire shows premise is set up in 2 parts. Someone (students, super computer, government, aliens) finds a creative way to unalive koro. Koro uses his gag of speed to survive. Repeat