| Yes. She stuck to her plan.   She focused on those students who volunteer to participate (which are very few) or do I have to say �who � are very few? 
 who should be used here - but check your tenses.
     She focused on those students who volunteered to participate who were very few. ( But this sounds a bit awkward) 
 You could say it this way:   She focused on those students who volunteered to participate, of whom there were very few. (sounds a bit formal)   or   She focused on those few students who had volunteered to participate.  (This is my preference if you don �t want to sound too formal.)       
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