The National Museum of American Art within the Smithsonian libraries has mounted a wonderful exhibit on Latino art over several centuries.
Check out their extensive Education section.
Another specialized art site is Asian Arts - exhibitions, articles and links for older students.
The National Museum of Women Artists has placed some rare works of even well-known artists like Frida Kahlo online – an opportunity to inspire budding young women who cannot visit in the real world.
For an introduction to architecture for high school students look at Great Buildings.
The International Sculpture Center provides an interesting introduction to sculpture.
The Los Angeles County Museum of Art has digitized a large portion of their collections, including exhibits. You will certainly find illustrative examples of many topics here.
Since access to computers and online resources, other than in community access in schools and libraries and kiosks, are limited to the upper 30% income level in homes, it is a boon to welcome this site. Now, universal access to the arts in many forms will be available.