I disagree entirely with you on that, and yes, I've lived, worked and taught in Thailand, and Thai kids are really polite, at least to me and the rest of the staff.
The nasty ones were spoilt kids from nouveau riche families where parents were too busy out making money to spend quality time with their kids, and teenagers climbing up the wannabe ladder.
The worst behaved kids I've ever met are either Pakistani or Maori, quarrelsome and violent lot.
Tantrums? Chinese kids take the biscuit (and anything else they can lay their hands on).