Buckinghamshire operates an "opt out" system for the 11+, whereby children at Bucks state primary schools are automatically entered for the test unless their parents actively withdraw them from taking it. Children at private schools and those attending schools outside Buckinghamshire must actively apply to take the test.
Registration is required for all children attending Partner Schools and out of county schools). Children attending Bucks state schools do not need to register.
All this is correct at time of posting. Check here for any updates.