Why are Rin's friends still mad over the fact that he is the son of Satan in season 2? In season 2 episode 1, Rin's friends are still mad over the fact that he is the son of Satan.

Isn't this already over/resolved in season 1?
Yes, it is resolved in Season 1 but Season 2 is not a direct continuation of Season 1.

To go into further detail: Season 1 of Blue Exorcist becomes anime original starting from episode 17. There was a lot of backlash from the fans and if they wanted to make a Season 2 it would be impossible to continue from Season 1's end. So, the studio completely ignored all the parts from Season 1 that deviated from the manga in Season 2.