Throughout the time I was on teaching practice, my mentor in the English department was pregnant. She gave birth the very week I finished, and it always seemed to me that she was giving birth twice - once to her baby and once to me as a teacher. I sometimes think she wondered who was causing her the most trouble.

Each morning, I would talk through my lesson plans through the door of the toilet while she brought up her breakfast. I would step in to cover for her when lessons looked threatened by an urgent need for the loo. It gave team teaching an exciting new edge.

We completed my end-of-course evaluation while browsing over pram covers in Mothercare. “The thing about being pregnant,” she once said through the loo door, “is that everyone tells you that you ought to be happy all the time, and really, you’re pretty miserable.”

Later that year, I knew exactly how she felt. My first few months of teaching were among the most miserable I can remember. And what made my feelings of incompetence, inadequacy and general confusion even worse was that everyone was telling me I had finally achieved my vocation.

I should have been on top of the world. You have to have this sense of vocation to be a teacher. You don’t do it for the money. You don’t do it for the status. You don’t even do it for the chance to eat school meals every day. You do it because, at some cursed hour in your life, you were struck by a bizarre need to communicate to children how wonderful your subject is, and you are driven by the absolute conviction that it will somehow make their lives better. Or alternatively, like me, you watched Dead Poets’ Society at an impressionable age and thought that it might be nice to have all those gorgeous young blokes reciting you poetry in the dead of night.

In short, vocation is that magic thing that your lawyer, merchant banker and check-out operator friends envy - and, if you have it, everyone thinks your professional problems fade away.

Being possessed of the v-word means that, in the eyes of the world, you are illuminated by some kind of magical inner light which enables you to glide, Florence Nightingale style, through the indignities that the teaching profession may throw at you. You are empowered by the shining conviction that you were always meant to spend ours up a ladder fixing posters to walls, and filing in meaningless forms for exam boards, and generally doing the kind of things that must have hit the editing room floor after the filming of Mr Holland’s Opus.

It didn’t take me long to realise that, surprisingly enough, teaching is not like the movies. They don’t show Michelle Pfeiffer up all night struggling to finish her marking. They just showed her doing karate chops and subduing a class of hooligans with the magic of poetry. So don’t worry if you’re feeling depressed and overworked and misunderstood in your first few weeks or even months in your job. Your ideals are falling away. You’re losing your vocational virginity.

You’ll be pleased when you finally get rid of it. It will mean you stop believing all those daft things that you’ve seen on TV, and you’ll start to be able to build your career according to what you want and need, and not what Michelle Pfeiffer wants or needs. And definitely not with the stubborn conviction that all you lessons should end with children saying “we are your symphony”.

Losing your vocation does not mean that you can’t be a good teacher. It means you can be a realistic teacher, and you can allow yourself to be a person - the kind of person who admits it’s okay to be tired and it’s okay to hate marking and, yes, it is okay to occasionally hate children.

Most importantly, you must not hate yourself. Great sins are committed in the name of vocation, the kind of sins that make you feel you have to put up with a load of rubbish without telling anyone or asking for help, because teachers can never admit they’re not coping.

When I finally went to my deputy head and told him that things weren’t exactly hunky-dory in the teaching department, I thought that he was going to think I was failing in some way. Of course he didn’t. He probably lost his vocation ages ago, and he helped me see that it was okay to lose mine.

Think of it like shedding a skin. You may feel a bit fragile at first, but it’s a vital part of the toughening-up process. Once that vocational veil has been ripped away, you’ll be ready for your second birth as a teacher: the one when you wake up in the morning and really know that you’ve made the right decision.

Gemma Warren teaches at The Latymer School, north London. She is a columnist in Friday magazine. e-mail: gemmaw@callnetuk.com