My colleague, Pat, is knocking on a bit now and doesn't quite have the energy and exuberance she used to. But in her day Pat was a fantastic teacher.
Pat came into the job when I was still at school. During that time she has seen policies - and government ministers - come and go, and carried on regardless as the whole education system has changed beyond recognition.
In the past few years, however, Pat has started to show her age: she moves around the place a little more slowly and the sparkle in her eyes has all but vanished. Her subject knowledge is still second to none, but rather than delighting in the challenge of inspiring children to share her love of her chosen subject, she now seems impatient, cranky and frustrated by a generation whose attention span seldom lasts longer than 140 characters.
Pat's lessons used to be full of passion and energy but these days she struggles to get out of second gear. It's clear that she's content to cruise until retirement. When I first arrived at the school as a young teacher, Pat's warmth and humour were legendary and she was adored by the kids and respected by her colleagues. These days, however, her disdain for the job and, worse, the pupils is thinly disguised. As a result, timetabling has become a nightmare as I seek to place her with classes where she will make the smallest possible negative impact.
Senior management is more than aware of the problem and is keen to bring matters to a head, rather than wait a couple more years before Pat can retire on a full pension. Part of me is tempted to go along with this plan to slap Pat on competency proceedings and bring in a younger, more energetic and - let's not forget - cheaper replacement.
But two things are holding me back. First, Pat has devoted the majority of her adult life to teaching and for the bulk of that time she's been bloody good at it. Does she really deserve such an undignified end to her career simply because Father Time has caught up with her and the world that she inhabits is barely recognisable? Having spent more than a few sleepless nights examining my conscience, I don't think she does.
My second reason for protecting Pat is altogether more selfish. You see, I'm scared that one day, not too far down the line, I'll be just like her. Changes to public sector pensions mean that I, and thousands like me, will be forced to soldier on until we're pushing 70, while living with the constant fear of being led off to the knacker's yard - on a drastically reduced pension - the moment we show the slightest sign of slowing down.
It's not a future that I'm looking forward to, so in the meantime I'm going to keep doing all I can to cover for Pat.
The writer is a head of department in the North East of England. To tell us what keeps you awake at night, email email@example.com.