Sorry, Frank!! You are right - I got muddled. You are right about cheeky Welsh humour - that's a good way to put it.
Oh Brummie, that is rather touching (even though your wife is not W elsh!) It is a mistake to try to lump Welsh girls together, too, because there are different cultures in Wales like everywhere else - valley culture, Vale of Glamorgan posh, etc. All good, all different.
Sue, you reveal the other side of the picture. Perhaps receptionists get defensive because some people are rude and then come across as rude themselves - or some of them, anyway.
As for the behaviour of those darling children, Chica, I can speak from my lifetime as a teacher. Write to the Head, giving dates and instances. Say roughly how many children are involved. Unless she is useless, she will do something. There are teachers on duty at lunchtime who could patrol that part of the field and if necessary, they could even make it out of bounds. Children who are little angels at home sometimes turn into little when they are unsupervised and feel they can get away with giving cheek to a stranger. If you still get trouble, the school should have a photograph of evey child and you could go there and identify them. Get the school on your side by being firm but utterly charming to the head and senior staff and let them know that you have entire confidence in their ability to deal swith this. (Yes, I know...) If you don't get satisfaction, try writing to the chairman of the governors or the Director of Education. Be charming at all times - you don't want to be categorised as a difficult person making a fuss about nothing. If you shout at the kids, you could provoke them to a trial of strength - children only obey someone they know has authority and with whom they are in a relationship like teacher/pupil (mpost of the time).
I could now engage in a rant about thew bahaviour of children. Believe me, I am tempted.