Relationship between social psychology and educational psychology
No, social psychology is not a subfield of educational psychology. Social psychology is one of the basic disciplines of psychology (like e.g. personality psychology or developmental psychology), whereas educational psychology is an applied discipline (like e.g. clinical psychology or industrial/organizational psychology). A sort of standard definition of social psychology is, that it is the 


  the study of the manner in which the personality, attitudes, motivations, and behavior of the individual influence and are influenced by social groups. (Merriam-Webster, 2013)


This means that as soon as there is some sort of social interaction involved in an educational setting, both disciplines are concerned.

References:


Merriam-Webster. (August 2013). Social Psychology. Retrieved from http://www.merriam-webster.com/dictionary/social%20psychology