Incorrect, it is entirely legal and permissible as long as it is limited it it's literary importance.  the problem is, these classes are rarely limited to this.   I had one in high school back in the late 70's.   The teacher was an English teacher, but she was also the wife of an evangelical minister.  Mostly the class was related to literary devices, but every so often it would stray over into theology.    that is where the problems start!