Not necessary, because even if other schools can be appropriate for study, that doesn’t change the fact that the researchers conducted a study and found certain results. They are allowed to interpret those results as supportive of a conclusion, even if there were other schools they could have studied. In any case, we’re not told the kind of schools that were part of the study, so we have no reason to think that the schools were limited only to large elementary schools.

Not necessary, because even if teachers devote different amounts of attention to different students, it can still be the case that the average amount of time teachers spend with students increases as class size goes down.

Not necessary, because the number of teachers isn’t relevant to the reasoning. In any case, it would make sense for a reduction in class sizes to be require MORE teachers, not fewer.

Necessary, because if this were not true — if the level of engagement in the learning process does NOT correlate well with students’ average grades — then the fact students’ grades didn’t change in the study would not support a conclusion about the level of the students’ engagement.

Not necessary, because the reason parents support the law is irrelevant to the researchers’ argument. The researchers’ argument is based on a study, not on why parents support the law.