prep school vs sixth form

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. 

sixth form

noun
  • In England, the final two years of secondary school, during which students are about sixteen to eighteen years of age. 

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