Primary schools vary enormously in their attitude to the 11+. Some actively support it; some are neutral; some discourage it as divisive. Knowing where your school sits helps avoid friction.
The short answer
In broadly grammar-supportive areas (Kent, Bucks, Birmingham), most primaries are pragmatic about the 11+ — they may not actively prepare children but they will not get in the way.
The longer answer
In areas with no grammar tradition (most of the North-West, large parts of London), primary schools may be openly sceptical and reluctant to support preparation. Your child should not be drawn into school-vs-home tension.
Talk to the headteacher (not the class teacher) about your plans early in Year 5. Most heads will be supportive in principle; the pragmatic question is whether the child can be excused from a small amount of homework during the autumn-term test window.
What experienced parents do
Avoid using the school as a delivery channel for 11+ practice. Schools are not set up to provide format-specific GL or CEM preparation; expecting them to is unrealistic and can damage relationships.
What to avoid
Avoid: telling other parents at the school gate that your child is sitting the 11+. The dynamic is rarely healthy, and the comparison-and-anxiety culture that develops helps no-one.
Practical next step
Maintain the child's normal primary-school routine throughout. School friendships, extra-curriculars and holidays should not be sacrificed to the 11+ — children who keep their normal life perform better, not worse. A small, deliberate action this week is worth more than a grand plan for the year ahead.