grammar school vs sixth form

grammar school

noun
  • A secondary school that stresses academic over practical or vocational education, until recent times open to those pupils who had passed the 11-plus examination. 

  • Elementary school. 

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 grammar school and sixth form occurred in a corpus of books? (source: Google Ngram Viewer )