Excuse my language but I usually used "*sshole" as a code name for most of my teachers. /s
There is a teacher at my school who gets annoyed if boys and girls are sitting next to each other and accidentally bump knees... Our 'codename' for her is a literal 'coocoo'. She hasn't figured it out... Yet.
Another good codename is Pacman, a book/movie/tv character that is mutually hated by all in the party (eg. Umbridge), the toad, the shrimp. Be imaginative. The more creative, the more fun.