Dozens of Boston-area college students took advantage of the once-in-a-lifetime opportunity to make the 20-hour bus ride down to Selma, Ala., Friday to participate in the 50th anniversary of the pivotal “Bloody Sunday” march.“This trip gives young people a moment of reflection in terms of what happened 50 years ago and what is happening now,” said Tony Van Der Meer, a professor in the Africana Studies department at UMass Boston, who helped organize the trip that drew a crowd of 50 students.