I am not a teacher, by the way.
The antecedent of "who" is not "one", but "girls". "One" is not the subject of the verb. The actual subject is "who". It is, therefore, plural. If we turn the sentence around in our minds, we will end up with the correct verb: Of the girls who are missing, Jill is one of them.
Michael Swan says in his Practical English Usage (529.1) [3rd Edition] that "strictly speaking, a plural verb is correct" in one of+plural noun+relative clause structures. Frederick T. Wood, the author of Correct English Usage [1987 edition], holds the same view. Patricia T. O'Conner (in her "Woe is I") [pgs. 60-61: 1996 edition] suggests "If the that or who comes before the verb, it's plural; if not, it's singular". She gives examples like: (1) He's one of the authors who say it best; (2) One of the authors says it best.
Reader's Digest's "The Right Word at the Right Time" agrees with the above opinion.