selective mutism

:  an anxiety disorder of childhood characterized by consistent failure to speak in specific social settings (as at school) despite having the ability to speak normally in other settings (as at home) If your child talks a lot at home but clams up at school—not uttering a word or even a laugh—she may have selective mutism, an extreme manifestation of anxiety.—Scholastic Parent and Child, February/March 2006—called also elective mutism