I was reading some comments from parents on a forum recently about their experience of eleven plus mock tests and how difficult they are.
There was some debate on whether this was a good thing or not, since some felt it made their children worried. Others thought that it was helpful to have a hard test so that the real test would not seem so challenging. Most people said that 50% on these tests was a good mark and that it seemed consistent across providers.
One reason for making these mock tests hard is because it tends to spread out the student marks so it is easier to see a different between the best and less able students. If the test was too easy you'd find close bunching of marks at the top end and it would be difficult to have clear separation. At it's worst, you could find many students have full marks and would all have the same rank!
It's important not to worry too much about how hard the paper is, but instead focus on the rank of your child. This will give you an idea on how your child compares to others and determine whether a grammar school place is a realistic option.