What is the name of the phenomenon whereby someone steps-up and others follow? Example: In class, when a teacher proposes some unreasonable assignment, students hesitate to express their objection until someone steps-up and then others find courage to talk.

I've tried searching for the term for this phenomenon but I don't know what I'm looking for exactly.
Your example of no student willing to object until one objects is analagous to Asch's conformity experiments. To quote the Wikipedia article:


  In a control group, with no pressure to conform to an erroneous view,
  only one participant out of 35 ever gave an incorrect answer. Solomon
  Asch hypothesized that the majority of participants would not conform
  to something obviously wrong; however, when surrounded by individuals
  all voicing an incorrect answer, participants provided incorrect
  responses on a high proportion of the questions (32%). Seventy-five
  percent of the participants gave an incorrect answer to at least one
  question.
  ...
  
  The unanimity of the confederates has also been varied. When the
  confederates are not unanimous in their judgment, even if only one
  confederate voices a different opinion, participants are much more
  likely to resist the urge to conform (only 5–10% conform) than when
  the confederates all agree.