"And if they do badly," he said, "everyone blames the parents."
It was a joke, of course. But I do wonder sometimes how much influence parents have.
In the Mail on Sunday this weekend, headteacher Anthony Seldon said that all of us – parents, schools, the Government – should be working hard to teach young people the importance of good character.
"Traditional values include courage, resilience, loyalty, honesty, kindness, respect, responsibility and self-control," he said.
"To these we might add everyday qualities: politeness, punctuality, good manners. Studies show that these can be taught, and that they endure throughout life."
Hmmm. Sounds like a school assembly to me – brilliant in theory but impossible in practice.
I'm not being flippant. Really. It's just that the family environment is complicated. It holds people who would die for each other but who, on a daily basis, drive each other nuts.
So when it comes to how much parents can influence teenagers, I think it's important to remember that:
a) Your teenager will be shaped by a huge number of influences that are completely outside your control.
b) There is very likely to be a period of some years during which you and your teenager will disagree about nearly everything. Forget about enduring values like courage and self-control – you won't even agree about how to make a cup of tea.
c) If you're lucky enough to have a relatively stable family life without terrible traumas and life-changing calamities, your children's characters won't change much between the ages of 0 and 18. If you try to change them into something they're not, you're asking for trouble.
d) It's a teenager's job to reject all the things you hold dear – religion, The Archers, thermal underwear from Marks & Spencer.
What matters is how graceful you are in respecting their extreme antipathy towards your life's passions.
e) Your own behaviour is more telling than anything you can possibly say. If you're rude and mean to other people, they will be too.
I don't think, as a parent, you can set out to influence character. I think your job is much more straightforward.
Keep telling them you love them. Because that's a solid rock for them – and you – to stand on.
What do you think?