It’s much easier to judge aptitude if the students come in with the same knowledge base. Then you can watch who grasps the material quickly and can manipulate and extend concepts well as they think in front of you. But what if the student has been both taught and tutored in the subject you’re teaching before he enters your class. What you think is sharpness and aptitude may not be at all.
2/3 of the students in my older D’s chem class had taken the same class over the summer before school began. What are the chances that the kids getting recommended for AP Chem the following year fall in the 2/3 group? Pretty high. Teachers think they’re comparing apples to apples, but they’re not, or else know they’re not but test scores are test scores. Do you really think they’re going to say, “Hmm, that student may have earned a GPA that’s 2 points higher because he already took the class before so it was much easier for him, so maybe I’ll recommend the other student with a lower GPA instead.”