I've watched students in their mid-teens burst into tears and call themselves failures for their exam results. They're too young for that kind of pressure, and calling someone a failure (which, face it, is what the 11 plus exam does regardless of if they use those exact words or not) at such a young age will only be a self-fulfilling prophecy.
Whoop, didn't pass the exam, ey? Well, better take your dumb self to the dumb school with the other dumb kids.