Education is one of the holiest occupations in my view. I disagree with that teachers should be paid according to how much their students learn. Although teachers are important for students, the most important part for learning knowledge is students themselves.
Different teachers has different style, so it is hard to evaluate how well each teacher teaches according to how much students learn. As nobody would have the same character, teachers also have their own teaching styles such as humorous, gentle, strict, to name a few. Maybe some students like humorous teachers, while others prefer a gentle one. But probably the students of a strict teacher could give good results in their study, even though these teachers teach the same thing. We could not say that the strict teacher should be paid higher than the gentle teacher, since we know that the students have learned more because they were afraid.
On the other hand, how much or how well students learn depends on students themselves. In a school, it is easy to see that some students are with high marks, while some others have lower marks. A teacher is a person who direct students how to learn. Students should do much more works. The more attention students pay, the more knowledge they learn. The hard-workers probably get higher scores. Students" learning results depend on individual students" intellectual abilities and the time and energy that they put into learning. Sometimes it has nothing to do what how well the teachers taught. In addition, knowledge that students learn not only comes from the teachers, but also from many sources. Family influence, extracurricular reading and out-class teaching are some of the sources that can provide students with knowledge.
In sum, although teachers are important in students" education and learning, there are many other factors that decide how well students" learn. So it is unfair that teachers are paid according to how much their students learn.