A close second were the thousands of plastic cups arranged on the floor outside the sixth-form offices. For a while we forgave them the fact that no one could move along that corridor, let alone open the door, because each cup was filled with water and some had bright, fluorescent pink paint. Again, the time and effort involved here was impressive. Viewed from above, the effect was rather Antony Gormley. Clearly this year's sixth form are an artistic lot.
What kind of world awaits them? I wish them long, lazy sun-filled days. Yet it appears from the latest headlines that they will get a grey, depressing, storm cloud-laden horizon. Economic doom and gloom. Costs up and confidence down. Financial meltdown. A world of scrimping, saving, bagging a bargain, growing, knitting or sewing your own. Off into this depressing economic morass they and their youthful, energetic optimism must go.
Do they understand how to manage their money in such belt-tightening times? Probably not, if research from one leading high street bank is anything to go by. It found that 800,000 school-leavers thought an ISA was an iPod accessory. Another 600,000 thought it was an energy drink.
Later in the day there was consternation when we heard a Year 8 class had been kidnapped. This clearly wasn't as well-executed a plan as the lawn laying. They picked the wrong class and met with resistance, then found that trying to hide 24 12-year-olds in the Combined Cadet Forces hut was no easy task. Year 8 had decided it was payback time for the day before, when rumours had circulated that the sixth form had doctored their doughnuts with laxatives. It was only ever a rumour, but the loo was full of pupils clutching their stomachs and vowing revenge.
My final lesson. At their request we had millionaires shortbread and I made cups of tea - a simple, grown-up treat in the classroom. I wanted to say something profound or moving, give last-minute words of wisdom about life, but nothing sprang to mind. I wondered if they would have the skills to cope in the world to come. Plant potatoes. Sew a skirt. Bake their own bread. I considered telling them Leonardo da Vinci's phrase that poor is the pupil who does not surpass his master. Then I realised they had already surpassed me, at least in artistic ingenuity. Instead I asked them how they covered the cost of the turf. Collective contributions, apparently. Ah well, why worry? If the worst happens, they can always become landscape gardeners.
Julie Greenhough, Teacher at a boys' secondary in London.