One year, my class was worse than ever, with no sense of responsibility for the resources of the classroom, so I decided to make it clear that supply was not endless in our stock room any more than it was on our planet. I did some shopping and went in on Monday morning equipped with my new weapon: tubes of blue pencils, each one recycled from a plastic cup.

I explained to the children my concerns about the way they were treating their pencils. We had a discussion about where pencils came from, and plenty of pupils were not able to trace the origins of these writing instruments to natural resources. Some seemed genuinely surprised that trees had anything to do with it. I used one of those “in-your-face” images of a forest with a big man-made hole to make the point that I felt enough trees had been felled for our writing needs and showed them the recycled pencils. “These were not made from trees,” I said, “they are made from cups that have been thrown away, just like some in this class have thrown away their pencils. Let’s see if we can look after these a little better.” Each one was labelled with a name and given out quite ceremoniously.

The difference was incredible. When pupils misplaced their pencil, there was concern bordering on panic. Partners would compare pencil length and there was far less ritual pencil sharpening. When one did get lost, there was genuine sorrow and resolve that the replacement would not suffer the same fate.

So have I purchased these pencils for every class since? No, I haven’t. I realised that the difference was not brought about by the pencil being blue or a novelty. The pupils changed their behaviour when they understood the cost of what they had. I do keep a supply of recycled pencils to hand, but I want pupils to care about the price paid by the planet for the resources they use, and I would hardly be doing that if my class believed that a plastic cup was more precious than a tree.

Peter Greaves is deputy head at Dovelands primary school in Leicester