This is an excellent powerpoint specifically designed to give secondary school students a basic understanding of 20th Century Classical Music.
This powerpoint includes a simple explanation of serialism with diagrams, and information about the compositional ideas of Shoenburg and Babbit.
It outlines the Futurist movement, and shows examples of futurist paintings (Giocomo Balla, Luigi Russola, etc.)
It discusses the life and music of Edgar Varese and Kristof Penderecki, with images of their compositions.
It explains and provides images of John Cage and information about the prepared piano.
I would suggest finding some music samples of music by each of the composers discussed, in order to enhance the lesson (as these are not included in the presentation). However, it would not be required in order to present the slides to the class, as this powerpoint is information-based.