Yeah, I don’t think understanding conditional reasoning is important on this question. Focusing on the conditional reasoning doesn’t really have anything to do with the question type- it’s just a matter of when it’s required to understand what’s going on in the stimulus.
Think about the negation of E: Qualified teachers could be persuaded to relocate in significant numbers to the educator’s region to take teaching jobs. If that’s the case, then the argument falls apart, for the only reason to think that reducing class sizes would probably not improve overall student achievement is due to the presumption that there won’t be enough qualified teachers. There’s already a shortage of qualified teachers, so if E had said “No qualified teachers could be persuaded to relocate to the educator’s region to take teaching jobs”- the denial of that way of putting it wouldn’t be enough to make the argument fall apart, for that would be consistent with ONE qualified teacher being persuaded to relocate, and that wouldn’t be enough to make the argument fall apart. That’s why E uses the language “significant”- the argument needs to be assuming that there aren’t a bunch of qualified teachers that will come over to the educator’s region.