Maybe this is too obvious. Rick Reilly (a writer with Sports Illustrated) did a piece on the four athletes that (it would appear) took over Flight 93 when terrorists tried to turn it toward Washington, D.C.—instead overpowering their captors and causing the plane to crash into a field in upstate Pennsylvania, all hands lost.
I am including a website (it is one of several, I believe) that contains the story of the four athletes, though this specific site concentrates on Mark Bingham. However Reilly's story is there as well. It is a piece I revisit each year when we remember 9-11, and it still gives me goosebumps.
So many people were lost, that the stories may be endless—for some, we will never know what they did in giving their lives to help others. However, I find that those people whose stories we know can provide very real and modern day examples of true heroes.
And for any young man or woman who has served in the war in Iraq or Afghanistan, certainly there are heroes there, and stories to tell. I'm not sure how in depth you will want your students' work to be, for there are some of these heroes whose recognition begins with their death, and there may only be local accounts in the newspaper or a magazine. But I would expect that perhaps in looking at these kinds of people, especially with things that have occurred in the students' lifetimes, the experience may also be more meaningful.
Other heroes, not as well-known as MLK, Jr., but still as brave and still inspiring catalysts for change, were the Freedom Riders, those who occupied lunch counters in sit-ins, those who marched from Selma to Montgomery, etc.—they were heroic. And then there were those who never asked to be heroes, but changed the face of America: Emmett Till, the little girls killed in the bombing of the 16th Street Baptist Church in Birmingham, the Little Rock Nine, the three Mississippi Civil Rights members murdered, among so many others...all were heroes—and never knew it. Then the question arises, what is a hero. Must it be someone who consciously decides to take a stand regardless of the sacrifice required? I'm not sure if this last group "counts" for this assignment, but heroism has many faces...it brings up an interesting question.
Good luck: great assignment.