“How long have you been doing supply for?” I asked the cover teacher as I filled her in on the delights of the afternoon that awaited her.

“Just a few weeks,” she said. “I left my school at the end of last term after 25 years. I thought I’d be there until I retired but the new head made it impossible.”

It wasn’t the first time I’d heard a story like this, and it’s a terrific waste. Within the close-knit community of a primary, longserving staff can make an impact that is incalculably valuable.

It made me think of my own primary school and Miss O’Rourke, who joined the staff straight out of teacher-training college and stayed put until she retired 45 years later.

To say she was a legend in the building would be to put it mildly. Terrifically strict but terrifically kind, the respect she commanded was palpable. Children talked of her classes with excitement and trepidation; her advice and support carried huge weight with new teachers, and aggressive parents turned to putty under her gaze.

For staff and pupils alike, it was comforting to know that teaching fads may come and go but Miss O’Rourke would never change. She could spot an untucked shirt or missing tie from 100 paces and her insistence on perfect manners and perfect handwriting set the tone for the entire school. “This school is in my blood,” she told us, as several generations of staff and pupils crowded into the school hall for her leaving do.

Career for life

I hope similar speeches are still made now, but I worry that some of them are being made years earlier than they should be.

For my parents, teaching was a career for life. That was just the way it was. When my Dad retired, the school held a week of retirement celebrations; when my Mum left, they named the school library after her. No one pretends now that most teachers are in the job for life. The fact that the most popular training route is called Teach First shows how the mindset has shifted.

Even if you want to stay in one place, the choice might be taken out of your hands. I know of at least two schools where a new head has swept in and cleared out an entire staff (and with them all the school’s traditions and ethos) within a couple of years.

Putting aside the simple fact that now more teachers are leaving the profession than joining, driving out experienced staff is something you can ill-afford to do; a school loses more than just a class teacher when a long-serving member of staff decides it’s time to go.

It seems entirely wrong that supply agencies are filling up with long-serving classroom teachers who are looking for a better work-life balance or who felt they were no longer valued in the school to which they had given so much.

So if you are a head lucky enough to have inherited your own Mr Chips, make sure you do everything in your power to stop them from saying goodbye.

Jo Brighouse is a pseudonym for a teacher in the Midlands. She tweets @jo_brighouse