Because if men are from Mars and women from Venus, then parents are from Pluto and teachers from Galaxy Five Nine Zero on the far side of Asellus Borealis.
Far from approaching each other in a spirit of willing partnership, these groups do not even speak the same language. Take the Midlands comprehensive where parents recently raised queries about seemingly erratic marking. The school's response? Three incomprehensible, close-typed sides of A4 lecturing parents on the differences between "formative" and "summative" assessment.
Or take the many parents who contact the weekly educational advice column I write for another newspaper. The school won't listen, they write. How do I get them to take me seriously? What else do I need to do to make them see there's a problem?
Of course, parents are hopeless at home-school relations too. During my own 20 years as a school parent I mishandled just about every serious discussion I ever had about my children's schooling. I got worked up about things that didn't matter, and failed to get anything done about the ones that did. I nodded through the jargon of parents' evenings, then came away kicking myself for not asking for the information I really wanted. And I never properly learned that my children's version of events - or their teachers' - was rarely the whole story!
The problem for parents is that schools make us nervous and our children's problems make us upset. We may psych ourselves up to talk to a teacher, and that can make us aggressive, defensive, anxious or tongue-tied. Or we don't want to seem too pushy. We want to be liked, and want our children to be liked, too. So we go marching in, intent on action, only to come meekly out again with just a fistful of excuses to show for it.
Meanwhile, teachers tie themselves in knots over parents. They are so used to talking to children that having a conversation with a fellow adult can be a real challenge. They resent parents who come in and make a fuss, and resent those who show no interest.
Young teachers are easily intimidated by sophisticated older parents, while older teachers greet all parents with ingrained cynicism. They know the "gifted" and "sensitive" children we talk about are usually run-of-the-mill boys and girls. And all teachers, whatever their age, are increasingly frightened by parents who threaten to sue them or duff them up. In short, teachers and parents come from different corners, and have different goals.
Parents want red-carpet treatment for their offspring; teachers want to keep their classroom show on the road. Parents get frustrated when schools don't take them seriously; schools grow hostile and defensive when parents make demands they think unreasonable.
All this means that, despite the cheery smiles at the Christmas concert, the basic underpinnings of the relationship remain suspicion, mistrust and antagonism.
The only way past this is to look to the basics of human interaction. New teachers need to be trained to build links with parents while new parents need to be required - or at least encouraged - to attend sessions on how to work productively with schools.
In both cases, the information should be positive, practical, and based on real-life scenarios. It should cover language and body language; how to set out a problem; how to listen; and how to manage both frustrations and expectations. It should encourage each side to see where the other side is coming from, and focus on finding ways forward.
Above all it should emphasise, over and over again, that when teachers and parents work together the benefits for children are enormous - but that this can only happen when both groups have the confidence to speak freely to each other, knowing they are on the same side.