A teacher is generally regarded by many an authoritative role in teaching activities and being a teacher requires solid academic ground in the subject which the teacher specializes in. Therefore, teachers' teaching performance should be assessed by teachers, who have been trained in teaching methodology and thus are more professional in evaluating process. However, as far as I'm concerned, it is more useful for schools to collect information about teachers' performance and allocate rewards to those who perform well based on students' evaluations.
To start off, it is more reasonable and effective if students evaluate teachers' teaching performance. Because students are the direct receivers in everyday teaching activities. They are better acquainted with their teachers' teaching performance either in teaching expertise or teaching styles. They can judge whether their teachers' methods are effective for them or not in a long run. Whereas, other teachers are not able to know well the teacher's teaching performance by attending only one demo class of the teacher being evaluated. Since a teacher's overall performance shouldn't and can't be assessed merely through several class demonstrations, evaluation for teachers' performance from their colleague teachers shouldn't be seen as reasonable and valid enough. In addition, teachers are supposed to respond to students, not to their colleagues nor supervisors. An experienced teacher should prepare teaching materials and give assignments based on students' learning abilities. Teaching adjustment is always necessary in making lesson plan. This process, however, may conflict with the standard curriculum plan that schools assign. In this case, the teacher will probably choose to conform to the standard lesson plan to appeal to his or her peers and the school's authority if his or her teaching performance is evaluated by them. Therefore, it is essential to design a valid evaluation system to incentivize teachers to respond to students rather than others.
Secondly, students are more inclined to give more objective evaluations compared with other teachers. Students tend to make assessments according to the teacher's teaching expertise and popularity among students, while other teachers might give unfair evaluations due to the competition existing among them. If one teacher takes the lead in evaluation ranking, his or her peer teachers may seem uncompetitive by comparison. Biased comments might be given by other teachers out of defenses for their own interests. So, if schools reward teachers based on other teachers' assessment, there may be more chances of controversy arisen regarding to the fairness of the process. Also, this may lead to a detrimental trend that teachers will no longer respond to students' well-being, but concentrate more on how to make friends with their colleagues and supervisors, which will further increase the bias and controversy of the evaluation process.
In all, it is necessary for schools to adopt both students' and teachers' evaluations in order to be objective for assessing a teacher's performance. However, it is unreasonable to focus on the latter. It is more useful to evaluate a teacher's performance by students because they have more reasonable and objective considerations compared with other teachers.