With stakes and string lines we set out the site.
We popped the hose over the stakes and created a dome.
We fixed horizontals using a spacer to speed up the process.
The weight of the timber and laserlite flattened the dome.
Which was fixed with a central post and some angle bracing.
Thanks to Shay, Joe, Gabe and Zephyr for your help, and to Paul, the boys' teacher who said it was OK they stayed out of class to help. Yes, a rare and wonderful school.
I made a smaller, but similar structure at home last autumn using reclaimed materials. It's much smaller so you couldn't conduct a class in it. Nonetheless, any size greenhouse is helpful in cool climates, to keep food production going through the winter.