Every so often, newspapers trot out an article filled with amusing quotations from the school reports of famous people. You know the sort of thing: John Lennon was on the road to failure, Peter Ustinov's originality had to be curbed. Their underlying message is loud and clear: don't trust what your teachers say because they'll have to eat their words.
Each year, when GCSE and A-level results come out, however pleased we are with those people who have done well, there's someone who'll delight in telling us they did better than we said they would. When predicting grades, it's becoming increasingly difficult to resist the pressure to forecast an A when we're convinced a B is more likely. There's always a voice at the back of our minds: what if they prove us wrong?
Of course, it would be churlish to begrudge students their success, but we all know how frustrating it is when someone who's bright but lazy puts in a stunning performance and ends up with a top grade after years of idleness.
If you're unlucky, they - or their parents - will confront you in the supermarket to gloat. Sometimes it's the worthy sloggers who surprise you, squeezing over the grade boundary by sheer hard work.
Before you know it, you end up feeling that your professional judgment has been shot to pieces. You might as well predict everyone an A* and turn your lessons into one continuous group hug.
Is there a point where optimism becomes irresponsible? Every autumn, I spend hours talking to sixth-formers about their university applications.
And, every year, some set their sights too high. Maybe they've always wanted to be a vet; maybe they're desperate to apply to Oxbridge. And maybe it's worth a try. But what do you say if you really don't think they're going to make it?
It's hard coping with not being able to achieve your heart's desire. It happens in all spheres: the actor who never gets the lead roles; the athlete who misses national team selection. Some will argue that it's better to try and fail than never to try at all - and thus never know whether you might have succeeded.
Failure is harsh when you're 17. Telling someone their dreams may never come true is difficult. You're opening yourself up to all kinds of criticism: that you're lowering expectations, that you're trampling on fragile egos and injuring self-esteem. But, at some point, we all have to come to terms with not getting what we want.
This year, the experience has been particularly poignant. After several years of trying to start a family, my partner and I have concluded that it's time to call a halt; the prospect of agonising months of IVF treatment is too horrific, the odds of success too low. If you've ever faced fertility problems, you'll know there's a rhetoric of never giving up: that if you keep trying, one day it will happen. But some of us will never get a turn, whether it's getting pregnant, getting into Oxford, or being selected for the World Cup squad. And perhaps it's wrong to let people think otherwise.
For us, there's the prospect of a happy ending. We're applying to adopt; we've accepted that our family is going to be achieved in a different way, and making that decision has been hugely liberating.
But it's made me wonder whether there's a downside to the raising of aspirations; whether we need to be more careful in what we encourage our students to hope for. We can all dream, but you have to be realistic, cut your losses and make the best of what you've got.
The writer, who wishes to remain anonymous, is a teacher in Lincolnshire