I assume this requirement does not appear in the published admission arrangements?
If the child qualifies in the 11+, and is offered a place (either because there are sufficient vacancies, or as a result of applying the published oversubscription criteria), no one could lawfully take that place away, unless there has been cheating (e.g. an 'address of convenience').
There is an argument that over
-tutored children might flounder in a selective school, but that is a situation that would need to be looked at in the light of their performance after
entry (not because they have been tutored).