There are a lot of wonderful resources put online by other teachers & organizations for students. These sites are great for sight reading and also are a good resource for holiday music or simplified versions of something a student may wish to learn. 

http://www.gmajormusictheory.org/Freebies/freebies.html

 www.music-for-music-teachers.com/beginner-piano-music.html

http://makingmusicfun.net/htm/printit_piano_sheet_music_index.htm

http://www.christmas-carol-music.org/

http://www.easysheetmusic.com/
(Limited to 3 downloads a day without membership)

http://www.8notes.com/piano/