When I started teaching, an “old sweat” said: “Parents can look and sound nice, but when push comes to shove they will stitch you up. Be polite but keep your distance - and never trust them.”

I politely listened and promptly forgot the advice, filing it under “Embittered Colleague”. Five years later I was an experienced and confident practitioner. I thought I had become a pretty good judge of character - but I was heading for a fall.

My worst parent was in many ways a very good parent. She was a stalwart of the PTA, a governor and incredibly supportive. I thought I would cultivate her: after all, you never know when such a well-placed and respected ally could come in useful.

So I began to give her little snippets of insider news. Nothing malicious, just some candid observations about the way the school was run. In turn she listened and nodded sagely at my views, thus soliciting more of them. At some point I strayed over the line that divided the candid and the plain stupid, and criticised the head in a throwaway manner. But, hey, I was among friends, so who cared?

Some months later I applied for a promotion. The received wisdom in the staffroom was that I was the nailed-on favourite. The interview was a dream. I sat back and awaited the inevitable confirmation of the appointment.

I was gutted when I didn’t get it and asked for feedback from the head. Her comments were a limp justification of the decision and I left the room barely able to keep the door on its hinges.

It was only when I heard some weeks later that the head’s tennis partner was none other than you-know-who that the penny dropped.

The writer is a deputy headteacher in Brighton.