What could it be?

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Across
  1. 5. During summer we go on
  2. 9. for the school musical
  3. 10. the opposite of boring
  4. 14. a dark day
  5. 16. cold x 3
  6. 18. sometimes school is
Down
  1. 1. not the same
  2. 2. At school sometimes I'm
  3. 3. Russia to Germany
  4. 4. Switzerland is
  5. 6. Skydiving is
  6. 7. question
  7. 8. opposite of more
  8. 11. Before a test I feel
  9. 12. he...TV
  10. 13. opposite of most
  11. 15. an instrument
  12. 17. write a