Since I retired from proper teaching two years ago, life has been blissfully free from anything that is liable to keep me awake at night. I say “proper” teaching because I cannot give up entirely and, much to the incredulity of some of my former colleagues, I do supply teaching when it suits me. It keeps me in touch with young people, for, although my friends in the University of the Third Age are lovely people, they are, by definition, old.

Being a supply teacher means I have the joy of not being burdened by inspections, unrealistic expectations, bullying management and the looming simultaneous introduction of two major new sets of exams. Life as a supply teacher is not always a bed of roses - not often, in fact - but the children frequently make me laugh and never make me cry. They certainly do not disturb my sleep.

But now I am worried that my days in the classroom may be coming to an end. I have heard about - although not yet experienced - schools introducing lessons that last 160 minutes. I read about one such school in TES - it had introduced the super-length lessons to improve behaviour in the corridors.

At this stage of my career, I take a largely academic interest in educational developments, secure in the knowledge that I am not going to have to get to grips with yet another exam syllabus or fret about another appraisal. But 160 minutes, for any teacher, is too long. Most of the schools I teach at have a five-period day of hour-long lessons, and that is about as much as we can all manage, students and teachers alike.

Supply teachers are generally left worksheets or textbooks, and classes routinely manage about 20 minutes before they start getting restive. The remainder of the lesson calls upon all of one’s classroom management skills. So how on earth do people keep students engaged for nearly three hours? I am looking at it from the perspective of cover but I cannot believe my permanently employed colleagues would feel any happier.

I am by training a modern foreign languages teacher, a subject where success is best achieved by regular, short spells of exposure to the language. And I spent the whole of my teaching career opposing the lengthening of lesson times - to no avail, since behaviour in the corridor is regarded as more important than teaching and learning.

I do, in fact, have some sympathy with the corridor issue because many schools I teach at have passages that are downright dangerous at the change of lessons. There is one where I cower in doorways rather than risk being trampled.

But, however serious these problems might be, lessons of 160 minutes cannot be the solution. Or, if they are, I’m out.

The writer is a supply teacher in Bristol. To tell us what keeps you awake at night, email david.marley@tes.co.uk.