boards vs prep school

boards

noun
  • Examinations given for entry to college or to qualify for a profession. 

  • A hardcover binding on a book. 

  • A stage (as in a theater). 

  • Structure around a rink for ice hockey. 

prep school

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

  • 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. 

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