Believe you me, that kind of behaviour isn't confined to the middle class. The lower class parents are more likely to think that their child is the next x factor or BGT star though.
No, I've experienced it having worked in schools over the last twenty or so years in Liverpool and here in Sussex. Thankfully, the idea of a sports day where there are no winners or losers is dying a death.
I think the internet had been a massive negative in the last twenty years in terms of contributing and fueling the shrill outrage if something occurs that's is out of step with a person's paradigm. It is difficult not to join the baying mob.