Converse means opposite. More study time equals lower performance. This is of course, a paradox.

Let's work out what you are trying to say. 1. "Students are more efficient in smaller classes." This seems to be what you want to say. "Efficient" is not the best adjective to use here. It is better to write the sentence like this: "Students learn faster and better in smaller classes." 2. "Students learn faster and better when there are fewer exams during the semester." There you are. Those are the sentences you need. They are written in good English and your readers will understand them clearly. You could combine them to say, "Students learn faster and better in smaller classes and when there are fewer exams." 3. "When there are fewer students in a class, the teacher can pay more attention to each and every one of them." You were trying to play gymnastics with words, and it was not very successful, I am sorry to say. Please understand that simple English is the best English.