consider the following:
- LEVEL of the student: what can he play? what is is his capacity? or you may go a little beyond his capazity if you want to push him some more.
- what are some technical and musical NEEDS of a student? determine them and give them appropriate pieces for these needs.
- PERSONALITY: temperament, is the student cheerful (some Mendelssohn or Mozart maybe), passionate (some Russian or Spanish music), or serious (some German music like Brahms or Schumann) as shown in his/her exterior/interior, is he introvert/extrovert? etc...
- REPERTOIRE BUILDING considerations: competitions, recitals, auditions, etc...
- what does the student WANT TO PLAY? you may consider this too, especially for those kids who "don't" really take piano studies seriously.
hope this helps.