grade school vs prep school

grade school

noun
  • An elementary school or primary school 

prep school

noun
  • A private primary school which prepares its pupils for the common entrance examination most commonly at the age of thirteen, and subsequent entry into public school. 

  • A private or public school intended to prepare its students to get into prestigious universities. 

How often have the words grade school and prep school occurred in a corpus of books? (source: Google Ngram Viewer )