Dear Ben,
addicted to the show, love the builds....
can i make a suggesting for a simple build, many of your builds make use of your CNC, laser cutter or 3D printer which many people dont have and therefore cannot replicate at home or school.
my suggestion is a simple weather vein, made of balsa wood which could be cut using a craft knife from a template printed on A4 paper. glued together with hot glue. it should be sturdy enough and a challenge for you to build without the use of your specialised cutters and machines :-) and easly creatable at home or in school.
the electronics would work out wind direction and wind speed. much like the more expensive commercial products.
perhaps an arduino UNO or simular which again is available in the local maplins (bit like your radio shack) shops, the general idea is to build something that the children can replicate and get a simular result to you and be able to say "i built one like Ben did"