<p>My sense is that in modern usage either the singular or plural is acceptable for both of the sample sentences, but that the plural is the more correct form for purists.</p>
<p>Turn the sentences “inside out”, with the prepositional phrase first. This shouldn’t change the correctness of the plural versus singular choice. The resulting sentences are clearly awkward and noone would write this way. But the rewrite should help to answer the question at hand.</p>
<p>So for the first sentence:
Of the three people I knew well in middle school who still [write/writes] to me, Jason is one.</p>
<p>And the second:
Of the five girls who [bully/bullies] me in class, I will catch one.</p>
<p>I dropped the “only” in the first sentence rewrite because it made the sentence unnecessary awkward. But it should be possible to add it in a way that doesn’t affect the sentence structure.</p>
<p>In any case, as rewritten I expect that the “plural” form is easy to justify.</p>
<p>BUT there are situations where grammaticians may argue for the singular. One example that I’ve seen is this:</p>
<p>I am one of those people who wants others to do what I think they should.</p>
<p>The emphasis above is on “one – me” and the choice of singular would serve to stress that.</p>
<p>My opinion is that these writing questions are much too subtle to ever appear on the official SAT.</p>