“I want,” he cried, waving his invisible banneret and sword, “every school to decamp on July 5. I want the teachers to lead the kids to Edinburgh. I want to see all their buses, their blazers, their flags up there demanding that they tear down trade barriers to African farmers and write off the continent’s international debt.”

But then he added rhetorically: “What’s more important - learning geometry or remembering these two days all your life?”

Er... let us not answer that straightaway. As a rule I am all in favour of children being whipped out of school from time to time for exciting and inspiring activities, but I do so wish that he had not chosen geometry.

The phrase gave too much away: Geldof’s age, his pop-starry, bunk-off-school mentality, his groovily ignorant We-don’t-want-no-educashun generation. I wish the great man had said: “What’s more important, personal, social and health education classes or taking direct action for what you believe in?” or “What’s more important, getting home for your favourite television programme or trying to help Africa?”

But geometry?...oh dear. Wrong message.

A grasp of geometry, the root of all engineering practicality, might lead you on to do something far more useful for the developing world. Once your imagination and understanding has stretched beyond Pythagoras and crossed the pons asinorum, you might go further and further.

You might become an engineer, civil or mechanical, and an inventor. You might join a radical third-world aid movement like Engineers Sans Fronti res and be a real star in the fight for humanity, not just yet another fun-loving demonstrator surfing a wave of self-righteousness and getting hammered on cans of McEwans in some Edinburgh gutter.

Take heart, you dear geek who remains behind in geometry class while the cool kids go north. It might well be you, not them, who in this century develops appropriate technologies and brings wells and irrigation, bridges and transport and healthcare to transform the lives of millions. Geometry is good, geometry is cool. So are maths, chemistry, physics and biology and all the handmaidens of practical progress. Politics is not the only thing that solves problems; nor is protest.

Kipling said it all in his poem about Martha and Mary, in which the Sons of Martha are toilers doomed for eternity to transport and illuminate and keep safe all the dreamers and politicians descended from the idle mystic Mary, who merely sat at Christ’s feet and did no food preparation or washing-up at all.

As a paid-up arts-graduate daughter of Mary, I think we hacks and Geldofs should salute the sons and daughters of Martha more often, not sneer at them.

So do not pick on geometry teachers, Bob. Without them you would not even know what was going on in Africa, because you would never have been there.

The wings would have fallen off the plane on the way. Oh, and without physicists you would not be on television, or have made any recordings.

Perhaps it is time we hammered home this message in schools, as part of citizenship. It is all very well lecturing children on their freedoms and rights and telling them to vote and agitate, but perhaps we should urge on them more often their duty to become actually useful.

Perhaps instead of constantly saying that they should work hard to get qualifications and go to university and earn high salaries, which appeals solely to their self-interest, we should take another tack. Maybe we should routinely point out the actual uses of each subject to mankind as they learn it, starting the term with an inspirational lecture entitled Why Maths Changes the World or Geographers are Vital to the Future of our Species, or Chemistry - the Building Blocks of Everything, or even English - a Tool for Peace and Understanding.

Perhaps we should lay it out for them honestly: “Look, you’ve all got to do mathsbiologygeography - there will be wastage, we know that, we are aware that some of you will never take it further and joyfully forget everything we teach you. Fine.

“But the odds are that a few of you might get it. And on those few the future rests. Learning is a lottery - but the next Brunel or Einstein could be you.”

If Bob Geldof can light fires by making the little bleeders feel important to the future of the world, perhaps subject teachers should ponder on how to do the same.