Teen quiz

My kids had their joint birthday party recently and it was my turn to do the quiz. As they are both teenagers, I did a multiple choice where the answers are numbers from 13 to 19 inclusive:

Only one person beat the pass mark of 15 out of 30:

1. A square number
2. How many pounds in a stone
3. 1990s boy band named after a London area postcode
4. In what year are they planning to hold a referendum on EU membership in the UK
5. How many minutes long is "Voodoo Chile" on Jimi Hendrix's 1968 double album "Electric Ladyland"
6. Looks like a triangle
7. "The Answer to the Ultimate Question of Life, the Universe, and Everything" divided by three
8. "Surfin' USA" by the Beach Boys was set to the music of this Chuck Berry hit
9. House number where I…, C… and H.. used to live
10. Looks like the letter B
11. Hit song by One Direction
12. Number of stripes in USA flag (representing the British colonies that declared independence in 1776)
13. Atomic number of silicon
14. Average age of a soldier in Vietnam according to the 1985 chart-topping single by Paul Hardcastle
15. Last Apollo mission to land on The Moon
16. Atomic mass of oxygen
17. How many months in a leap year in the Hebrew calendar
18. Body-swap film starring Zac Efron and Chandler out of "Friends"
19. In 2015, Mark & A... celebrated their …th wedding anniversary
20. Players in a Rugby Union team
21. Square root of CCLXXXIX
22. How many digits did Horatio Nelson have when he died
23. Players in a Rugby League team
24. Casual/modern British restaurant staffed by disadvantaged youngsters trained by Jamie Oliver
25. Days between new moon and full moon
26. The date today
27. Twice a square number
28. How many UK Top Ten hits did ABBA have
29. How many days in "une quinzaine"
30. The sum of the first three squares (which makes it a square pyramidal number)