Would you treat 'who' as singular or plural in the following?
No wonder that the designer is one of those who has been approached for the design job at Schiaparelli.

I believe that who here takes those as an antecedent.
The excerpt is from the New York Times: http://www.nytimes.com/2012/07/03/fashion/dior-says-it-with-flowers.html
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Comments  (Page 3) 

That's the thing. It the first, 'is' is related to 'he'. In the second, 'who' is related to (or modifies) 'students'. These are as per the subject-verb concord.