Mental midget.

Mental midget:
 * a person who is narrow-minded or has limited knowledge or insight to anything outside his or her immediate life.
* basically someone who lacks education or higher thought. 
     (Source : Urban Dictionary )

Some nice expressions to refer to brainless people:

"Not the sharpest knife in the drawer"
"As thick as two short planks"
"He's a card short of a deck"
"That's a bolt without a nut"
"There's a cog missing in that wheel"

                           Dear students,
                                           Educate your brain,
                                           inspire your soul,
                                          stupidity is not sexy.

                                           :)

PS: Thanks to my friend Consol Santamaría for such a nice recommendation.