This EdWeek article describes the concerns of the NEA over the use of student outcomes in teacher evaluations:
http://www.edweek.org/ew/articles/2009/07/07/36nea.h28.html?tkn=RVUFKEwgmsmybxEE5%2BgAQfMcjKftpBdukBKW
The moral of the story is: Multiple Measures