A demagogue is a leader who obtains power by means of impassioned appeals to emotions, prejudices, and fears. How is Jack a demagogue?

He appeals to the fears of the boys as they think the Lord of the Flies has descended upon the island and has come, perhaps, to claim them.  He appeals to the emotions as he inspires them to hunt and then act in savage ways at the end of the hunt.  Even using Piggy's name as an insult helps Jack to get the boys prejudiced against Piggy to such an extent that he has their complete dedication to whatever Jack wants.