The Wizard of Earthsea is six book series centred on a boy named Ged. He is a wizard and the children in his home village began calling him Sparrowhawk because he loved to summon the birds with his early magical skills. Ged was finally ready to travel to the island of Roke, where he went to school to learn to become a better wizard. At the school, he made friends with a student called Vetch and made a competitive, pride-driven rivalry with a boy named Jasper. At a school lunch, Ged overhears Jaspers’ remarks about him, and because of their rivalry, he takes offence to Jaspers comments and in response, challenges him to a duel in sorcery.
Even foolery is dangerous in the hands of a fool. – Jasper (Chapter 3)
Jasper may not be the wisest wizard, but I think he is saying this to the importance of education as well as using power responsibly. Although it seems that his only goal is to shame Ged, his words do hold truth. I think he is saying that all foolery is dangerous, and it doesn’t matter whose hands it is in.