Harry sits opposite you sobbing violently, slumped in his seat like an over-risen sponge cake. He’s a five-year-old with a BMI that comfortably makes him obese. The other students have noticed his size and let him know they’ve noticed it. He has come to you for support.

“You don’t think I’m fat, do you?” he asks, his eyes bloated with tears. The truthful answer is, “Yes, Harry, you are worryingly overweight.” But if you have a heart, at this precise moment you will soften the blow. You will not necessarily lie, but there’s no chance you will tell Harry the full truth. That can come later with his parents. And thus does a teacher seemingly take the first steps towards becoming a hypocrite.

As soon as they step through the school gates children are told that lying is the worst of all sins, and yet teachers lie constantly. They artificially inflate their expectations, turn a blind eye to minor indiscretions, pretend they like parents and colleagues, and claim in all sincerity that this is the best class they have ever taught.

Pupils know teachers lie like this, of course. As innocent four-year-olds, they are told to be nice to everyone and never to lie. Philip Larkin expressed the impossibility of enforcing both rules together in his poem Talking in Bed. “It becomes still more difficult to findWords at once true and kind,” he wrote. We set our children up to fail and they realise it from the start.

All this appears to make the sentiment of this week’s lead feature in the Professional section problematic (Turning detective to uncover the truth). Teachers are supposed to be myth-busters, lie-lambasters, whopper-whackers. A key part of their duties is to build honest citizens. Yet how can they enforce a rule that they break themselves?

The truth is that they don’t break it, not really. Teachers’ lies are white lies. They fib to protect children, to manage situations and to encourage and facilitate learning. Lies of this kind are a necessary part of the teacher’s toolkit. Presenting the whole truth and nothing but the truth would, in my humble opinion, make you less of a teacher.

Of course, harmful untruths have no place in teaching and even white lies should be selective. The aim should be to postpone telling the truth to a more appropriate time (as with Harry), or for your white lie to have at least an element of truth (Not “You will get an A if you work hard” but “You could get an A if you work hard”).

In dealing with dishonest students, the rules should be roughly the same. There’s no hypocrisy here, just honesty - of a kind.

But where is the line between good and bad lies? Teachers have no crib sheet or standards guide to refer to here. In the end, it comes down to whether we trust them to be professional and sensible enough to make the right judgements about themselves and their students. There should be no doubt: teachers prove themselves to be more than reliable and up to the task every day.

jon.severs@tes.co.uk