prep school vs 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. 

primary school

noun
  • A formal, obligatory kind of school that typically begins with nursery school or first grade and ends at fifth or sixth grade. 

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