Teachers share their knowledge with students, to make their students to be successful in life. Teachers put equal effort to make their students bright. I think it is unfair to judge teachers according to how much their students learn. Here is my reasons and examples.
First of all, It is students self ability to score well on exams. It really depends on individual students. Like students individual thinking ability, how fast they understand the concept. For example, Some students seriously study to get good marks. But some other students just waste time on other activities other then studying. Or sometimes some students work hard but will fail to put full effort on exam may be because of exam tension. This is also one reason for low grade. For this reasons we can not blame our teachers and pay them less.
In addition, Sometimes some teachers may tempt to earn more money, by giving more marks to their students and earn more money. Or it may be other way, What if a student do not like his teacher and intentionally fails or get less marks to take revenge or show his disrespect. For example, we can not tell a sales manager, that if he do not reach his monthly target, he won't be paid for that month. This is unfair. No one accepts it.
Further more, Teachers get one hour or 45 Min's to teach a class. Teachers spend lot of time to get prepared before class starts. So it not teachers mistake if his/her student scores less. For example, My teachers used to take extra classes to couch us, but they never got extra money. Because its teachers job to teach us. After that everything depends on how students preform on their tests to exams.