Here's a fun way to get your students listening to classical music and to introduce the four musical periods (don't miss more ideas on introducing the musical periods here) -
Have your students listen to George Rochberg's Sixth String Quartet (Variations on Pachelbel's Canon in D). I love this piece - it introduces each musical period by playing Pachelbel's Canon in each style. It starts out Baroque, and then segues into Classical, Romantic, then Modern, and then eventually makes its way back to Baroque. Check it out!