I recommend the Royal Conservatory of Music (RCM) guitar series. The easiest music is in the book titled "Preparatory Level," although a bit of prior work in reading and technique is required. After that, the music gradually gets more difficult with grades 1-8. This graded repertoire gives a student a wealth of music to choose from at a level that is appropriate for them. There are plenty of renaissance, baroque, romantic, and modern pieces to choose from. At the end of each piece are footnotes detailing the source of the music. As merely one example, Carlo Domeniconi has written a fair amount of great music even at the preparatory level. When many students fall in love with the music, they are easily able to obtain more of a good thing that is outside the RCM collection. This collection should be in every teachers library. At least the first three books (preparatory, grade 1, and grade 2) should be in every students library.
You are welcome.
Dr. Todd Tipton, classical guitarist
Cincinnati, OH, USA (available via Skype)