When we tackle necessary assumption questions, we need to negate the answer choices we are actively considering to see what impact that would have on the stimulus. When negated, the correct answer choice will weaken the argument.
If we negate D, and this turns it into: Hiring more teachers would improve the achievements of some students if most/all teachers hired were unqualified.
What effect does this have on the conclusion that says this solution would probably not improve overall student achievement? None, actually. That's because saying that some students will improve with this solution could mean that only one student will improve. In that case, the conclusion in the stimulus still holds.