Is it true that if a child does not mark EVERY answer on the multiple choice answer sheet then the paper is null and void? (Buckinghamshire 11+)
What happens if they miss one out?
Nothing, no mark but no penalty either.
The advice is generally in the last min. or so, to fill in any unanswered questions on the sheet in a straight line (ie all a's or b's) because there is a chance of gaining a mark.
For Bucks specific questions you are probably better off posting in Buckinghamshire further down the page.