I am feeling rather hollow after a long day scooting from class to class at the school. The ever-churning sea of at best cheeky, at worst hostile faces gazing back at me across the science lab has left me frazzled. Hours of flying by the seat of my pants have drained my batteries.

But the head of department doesn’t know this. It’s 3.30pm and she still seems as bright as she was this morning after three cups of coffee from her on-desk cafetière. It’s clear from the kids’ behavior that you have to be fairly indefatigable to work here.

“You seem to be getting on all right - the kids like you. Would you be prepared to stay for a six-month maternity cover? It would really get us out of a hole,” she says.

It’s the question that many substitute teachers dread. Contrary to popular belief, not all of us are doing this mercenary work because we can’t get jobs elsewhere: most schools these days are desperate for a living breathing science teacher who is qualified in anything other than biology. For many of us it is a conscious choice. We are prepared to put up with the uncertainty, the lack of status and respect in exchange for a 3.30pm home time and no marking or responsibility outside the classroom.

Being offered a longer-term post is therefore a scary proposition. How much will I have to invest in my classes if they’re mine for more than 50 mins? Will I be held accountable for their lack of progress, the exam disasters and all the rest?

And I worry about behavior: as my classes have rarely seen me before I find they usually give me the benefit of the doubt for the first lesson. Would a second lesson with the same class be as good or would they stomp me into the ground?

But the schools will continue to try to woo me into longer-term fill-ins. I’m a chemistry specialist who can teach physics and maths, I am the proverbial gold dust. And I speak English. And I can stand upright.

It feels good to be in this position. When I was working in a permanent post, my school made me feel like I never measured up. The incessant tests never proved my children had made enough progress. My data-entry skills were never up to scratch. My hair was lifeless and lacked volume.

No, I’m happy as I am, I think to myself. For my mental health and for the good of my own children, I’ll stay on short-term supply for now.

So I lean back calmly in my chair, reaching up to loosen my tie slightly.

“I’ll think about it,” I say.