If your child is at present in the state system but intending to take 11 plus, Common Entrance or entrance exams for a private school they may be at a disadvantage.
They may very well be gaining excellent marks at their school, even top of the form, but they may not have studied some of the areas which private schools take for granted. This is especially true in English where knowledge of parts of speech and accurate spelling and punctuation are often given greater weight in the private sector. I find many pupils are not used to their written work being marked for all punctuation and spelling mistakes; they are often rather shocked by the amount of red when I return homework. Their teachers may concentrate more on the imaginative/creative content in order to encourage writing. This is not necessarily wrong – just a different approach. Teacher training courses often tell prospective teachers not to mark everything as it will discourage pupils.
Pupils who are already at prep schools and within the system will be more used to these standards. When I taught in a prep school, pupils of 8 or 9 were often tackling work at the same level as secondary first year. Scholarship exams for the most prestigious schools can be way above GCSE level in subjects such as English, Latin and French. There are many excellent books that can help your child prepare for 11 plus and naturally (!) I would suggest that some private tuition can be of great benefit. Coaching in moderation can help level the playing field for state school pupils in particular.