First term is almost over and you think you've done well. Not because you've got John doing joined-up writing, or Jessica reading aloud. You're young, you think you're still cool, you understand the offside rule. Yet, at the end of the day, you're still one of them. You've tried so hard not to fall into that trap, but there's nothing you can do to change people's perceptions - you're a teacher, therefore you're old before your time.
So, how do people turn into teachers? There you are, an apparently normal, well-adjusted human being, and suddenly you turn into someone who watches Newsround and collects the cardboard inners from loo rolls. This doesn't happen all at once. There's a period of transition during which you slowly stop going out mid-week - or, in some severe cases, at all - and start to develop the ability to do six things at once.
Transition has nothing to do with teacher training. In fact, you may have noticed that it's possible to go through the whole of PGCE without exhibiting any of the characteristics of a teacher at all. And some people show all the signs of a fully fledged teacher without having been anywhere near an ITT course. There's one like that in Year 9.
But at some point, everyone goes through the stage where they imagine what kind of teacher they are going to be. This usually takes the form of a series of what experts call negative aspirations. Would-be teachers are keen not to internalise the existing practice and culture of the profession. More precisely, they are determined not to turn into Miss Smith, who taught them in primary school and had a habit of telling her flock that "You'll thank me for this one day". As if.
People begin with the best of intentions. They are not going to comment on the kids' hair, or make-up, or idiosyncratic way of indicating disapproval. But it's difficult. How can today's 14-year-olds not know that something awful is totally pants?
Music is a minefield. There's nothing as lip-curlingly embarrassing as last year's pop, but it is inexplicable that even the brighter sixth-formers don't like Radiohead or Coldplay.
The root of this problem is self-image. Our view of ourselves is partly defined by the picture we see reflected in other people's eyes. You think you're a young person, barely out of college. They see you as a trainee fogey, aged around 130 and irredeemably naff, simply through choice of career. And it is complicated because teachers are authority figures - you have to find some way of dealing with that role. It's about negotiation of the boundaries, but if you're not careful you'll find yourself using exactly the phraseology you swore to avoid.
"Unless Amber's new drawing pencils are handed in this instant," you thunder, "you will all be staying in after school." And so the commitment to avoid group punishments bites the dust. Collective punishment may be outlawed by the Geneva Convention, but Amber is a sweet girl who was proud of her new pencils.
Uniform provides a fair amount of angst. You hated uniform at school and drove the teachers mad with endless permutations. And now the boot's on the other foot. "These are grey," says Sophie. "It says so on the label." "But it's not school grey," you say with self-knowledge about the absurdity of the conversation you are having.
Gradually, the sense of the ridiculous recedes and you become aware that this is a role that you play. This isn't the real you, but a mask you put on each day. You'll find it easier to adopt a sorrowful expression and tell a child that "You have let me down, let the school down and let yourself down". As the exams approach you will tell your Year 11 groups that "the next few weeks will be the most important of your lives". You may even believe it.
But you'll know that the transformation is complete when you set your struggling A-level students enough work to keep them occupied for the whole weekend. As they gape at you with disbelief, you look ahead to the results you know they are capable of - and you say: "You'll thank me for this one day".