Clue #27 was living-group-specific. It has been removed and the answer given.

Note: This is a cryptic crossword. If you are unfamiliar with cryptic clues, here is a helpful guide.

Across

  1. Made up license for peace and quiet (7)
  2. Announced herb used for cooking season (4)
  3. First, Ron gerrymandered constituents to become more powerful (8)
  4. Promote revolutionary drink (4)
  5. Pain and audible expression of shock (3)
  6. First signs of students' unexpectedly changing hours are similar (4)
  7. In Georgia, throw ball (5)
  8. Drop endless diet (3)
  9. Phone prison room (4)
  10. Initially bolt, fasten, and stop (5)
  11. James's last car shows blemish (4)
  12. Former first two of examples (2)
  13. Not as much of the French students' outsides (4)
  14. Air is half-frisky (3)
  15. Catapult concealed by fourth rower (5)

Down

  1. It's in Twist book (7)
  2. Bit of rain to fall? Allow (7)
  3. No, get beginner if lacking animated image (2,3)
  4. Origin of my metal is the opposite of 23 across? (4)
  5. Country is almost new, friend (5)
  6. Baby's bottom becomes a titular puzzle game character (4)
  7. Gene is weird-looking (6)
  8. Handwritten jinx has four (6)
  9. Price with a bank (5)
  10. Creep up on unstarted lecture (4)
  11. Soup from street? Gross (4)
  12. Judges wound, cutting off two heads (4)

Zigzag

  1. Unintelligence held by sign or ancestors (9)