The lowest point of my teaching career was also one of my finest. It was a lesson, some years ago, one mile underground at the grimy face of a highly fractured coal seam.
Senior pupils had asked me to organise a field trip they would remember for ever and so a chance invitation to visit one of Scotland's last-remaining deep coal mines was too good an opportunity to turn down.
We were kitted out with overalls, safety helmets and headlights and asked to sign disclaimers releasing the mining company from any responsibility. Getting to the coalface involved dropping down a mile or so in a rickety cage, jumping on to a conveyor belt loaded with coal and then crawling through 3ft-high burrows.
The heat underground was intense and the tunnels seemed to be solely supported by flimsy-looking timbers. Our elderly guide insisted on recalling terrible stories of "timber falls" which "smashed in poor miners' heads".
The burrows opened out into a small cavern where a team drilled and chipped at the coalface. The noise was deafening and it was a surprise, for pupils and teacher alike, to discover people working in such appalling conditions.
A lunch break was called and the miners joined in our short lesson encompassing geology, primary industry and the historical employment of children in mines and mills. Our companions served as a pertinent reminder of the ongoing brutal working conditions endured by many, often low-paid, people throughout the world.
This was a trip that made a lasting impression. And the former pupils I occasionally meet still mention it with a smile.
On the practical side, the experience gave the pupils suitable material for their exam essays and provided a degree of personal motivation to work harder to obtain better career choices.
One thoughtful pupil was inspired to design a "robot miner" as part of his craft and design project, "so that no human miner would have to endure the sort of conditions I witnessed at the coalface".
The group photograph of the pupils, every one blackened with coal dust, hangs on my classroom wall. Today's students think it is a photograph from the 19th century.
Schools now organise a wide range of interesting trips to heritage industrial centres, First World War battlefield sites and other thought-provoking destinations. I was reminded of our journey underground after hearing a group of parents discuss their children's forthcoming school trip to China.
Field trips are, more often than not, the most effective and thrilling learning experiences that schools have to offer. Destinations are becoming more exotic and interesting and, like our trip into the coal mine, provide the sort of educational activities that pupils remember for ever.
John Greenlees is a secondary school teacher in Scotland