A meticulously prepared lesson with PowerPoint and colour-coded handouts counts for nothing when set against the right shirt or length of skirt.
It's the kind of thing PGCE students should learn. After all, we all know that the popular image of teachers - leather elbow patches and all - isn't stylish.
I know a well-groomed appearance can give you the edge. It's just that I've been playing the dress-to-impress game for a while and it's starting to have undesirable side-effects.
It all started on teaching practice. I was finding the transition from student to teacher hard going. The students weren't much younger than me.
Suddenly that cocky "I've got a degree and have written 3,000 words on the finer points of Jane Austen's narrative style" air was a thin defence against the inability of intermediate GNVQ students to grasp the basics of memo writing. It didn't help when I was expelled from a staff toilet and told to get back to my lesson because my teacher would be wondering where I was.
In the face of such challenges, I did what any woman would do. I went shopping. A two-piece suit in serious grey did wonders for my confidence. I strode into my next class with poise and authority. I can't remember how the lesson went, but I did feel like a teacher.
And the habit has stuck. A couple of trouser suits and easy-to-iron tops in a variety of vaguely fashionable colours now serve as my working wardrobe.
I put on a jacket and it's as though a neon sign has appeared above my head: teacher on duty.
But dressing in a suit for too long can seriously damage your professional self-image. A minor milestone on life's highway is all it takes to make you realise the deleterious effect the wrong choice of clothes can have. In my case, it was my 30th birthday.
I don't know how I ended up revealing to my A-level class that it was my big 3-0. Anyway, I was happily giving the game away, imagining the flood of cards from adoring students, when a voice cried out: "Thirty! But then you're young, Miss." In that moment my fragile sense of self began to dissolve. I realised I'd spent six years cultivating an aura of responsible maturity, only to establish myself as an old frump.
I wanted to be seen as older in a reassuring, older sister way - but not old. The shock in my student's voice, which she realised too late, was a wake-up call. It didn't help that half an hour later we reached the moment in The Merchant's Tale when the protagonist declares that women over 30 are, to translate loosely, "as dried up as bean stalks and cattle fodder".
I couldn't have found a better way to demonstrate the meaning of irony if I'd tried.
Since then, I've found myself doing and saying desperate things. In class, I'll drop in a comment about the lyrics of the latest Ms Dynamite song.
I've braved the cubicles of boutiques to try on clothes I know are too small, with price tags that are too expensive. I've bitten my tongue when tempted to enquire whether Mamp;S does anything similar.
And for what? Are my students suddenly seeing me in a new light? Do they recognise that I may be a teacher, but I'm also hip and trendy? Of course not. What I've come to realise is that you never really feel like the grown-up, responsible, proper teacher you always thought experience would produce. Inside, we're all still that nervous student teacher waiting to be exposed for the fraud we know we are. We just get better at acting.
So, turn up The Archers and help me sew on those leather elbow patches.
Debbie Parker Kinch teaches English in London