One of the best situations for any teacher is to have a class where all the students learn at approximately the same level, but few of them are lucky enough to experience it. Each student has their own set of unique abilities to absorb information, and those who educate them are tasked with discovering their strengths and weaknesses. If they have a small class, the students will benefit by being able to receive extra help where they struggle. Those who are part of a larger class may have to turn to their family when they do not understand some of the subject matter.
While there seem to be no classes where all the students learn at the same pace, most teachers can at least count on the fact that the majority of students will understand the basic subject matter they are being taught. Testing gives an indication of how well the students are learning, and their teacher can modify lesson plans if necessary. While they do not expect all students to learn at the same pace, averaging out the grades is often their way to keep the class moving forward with their lessons.