When doing performance evaluations, it's always a good idea to have teachers complete a self-evaluation first. Then you can see what areas the teacher feels are strengths and weaknesses. Also, if a teacher gives herself high marks for something and you had planned on writing up as an area that needs improvement, you can plan on handling that area more delicately. Along the same lines, some teachers may see themselves as weak in a certain area, while you find their performance to be above average. Take the opportunity to let that teacher know about a job well done.
Here is a form you can use to perform teacher self-evaluations. You can download the .pdf and print as is, or you can download the Word version and change the wording to suit your setting.