My mother was an art teacher and her lessons included a wide range of topics going into art's history and philosophy like; Russian Constructivism, Futurism, Modernism, Impressionism, and a long list of others in high school. There is way to many people who think that art is just "drawing triangles" (According to one really alcoholic principal.) and shouldn't get treated as if it's valueless by likely ignorant people. Art depicts the very concepts of what a society goes through like Mussolini Fascism in Italy. It's an insult that there is to many people who understand little of it's value to society when everyday our lives have been affected by its influences on how we build our cities, how we value our families, the cry and outrage of the mistreated, the fury of a nation, and depiction of modern society.
Schools exist to provide a well rounded education. The arts permeate every aspect of our lives so not providing a basic grounding in them is a huge gap in education. Without an appreciation of the arts and the tools to analyze them we cannot make informed decisions. Someone who has been exposed to the diversity of ideas and opinion the arts provide is more likely to approach all aspects of life with a more open mind.
Schools should fund the arts because they help you with your life skills. Learning how to read music helps with your math skills, drawing helps with your creativity and your spatial awareness skills, and theatre helps with your social skills and the way you present yourself. I could go on and on about this. Whether you realize it or not the arts are part of our everyday life and kids aren't going to help with that in the future if they don't first learn about the arts in school.