"Classical art" refers to the art of ancient Greece and Rome. It is specific to this era. "Classic art" is a more generic phrase, referring to art of a simple, elegant style not greatly subject to changes in fashion or style. Classical Art is probably the phrase you are asking about.

Joe has the right answer. "Classical" often refers to Greek and Roman classics ("Classical Studies" = the study of ancient Greek and Roman literature, culture, language, etc.). In other contexts it can also refer to later periods, as in the phrase "classical music," which refers to music that is from the period between 1720-1850 (or music that follows the traditional style of this period). In general, "classical" will refer to something that is traditional and somewhat ancient, whereas "classic" describes the fact that a certain work of art (or style of art) continues to be popular across multiple eras or generations.