When is Harry acting heroically?

Harry Potter acts heroically at several points in Harry Potter and the Sorcerer's Stone. One example is in chapter 16. In this chapter, Harry begins to realize that danger seems to be connected with the pain he feels in the scar on his forehead. He explains to Hermione,

"'I wish I knew what this means! . . . My scar keeps hurting—it's happened before, but never as often as this.' . . . 'I'm not ill . . . I think it's a warning... it means danger's coming....'"

Harry then realizes that it's extremely unusual that Hagrid was able to get a dragon's egg at the local pub. He wonders if Hagrid might have revealed any information about Fluffy, the three-headed dog that was guarding the sorcerer's stone, to the stranger who gave him the egg. After asking Hagrid for some more information, Harry and his friends find out that he revealed to this individual how to make Fluffy go to sleep. (This would allow this person to get closer to the sorcerer's stone.)
Harry, Ron, and Hermione decide they need to talk to Professor Dumbledore. However, they find out from Professor McGonagall that he was called away minutes before. She tells the kids that they can wait for him to return the next day to pass on their supposed news about the sorcerer's stone (something students at Hogwarts aren't supposed to have any knowledge about). Harry and his friends think that the stone is in far more danger than Professor McGonagall does.
While Harry and his friends determine what to do next to try to protect the stone, Snape approaches and threatens Harry:

""Be warned, Potter—any more nighttime wanderings and I will personally make sure you are expelled. Good day to you."

Students are supposed to be in their dormitories after certain hours. Even so, Harry tells his friends a bit later,

""I'm going out of here tonight and I'm going to try and get to the Stone first."

At first, Hermione and Ron are not excited about this idea, but Harry reminds them of the danger they are facing:

"Don't you understand? If Snape gets hold of the Stone, Voldemort's coming back! Haven't you heard what it was like when he was trying to take over? There won't be any Hogwarts to get expelled from! He'll flatten it, or turn it into a school for the Dark Arts! Losing points doesn't matter anymore, can't you see?"

Harry continues to explain his plan for finding the stone by himself, but his friends will not let him go alone. They realize that a hero is always stronger with his or her friends and demand to go with him to find the stone and keep it away from dark wizards.
Harry shows heroism throughout chapter 16 as he risks his ability to stay enrolled at Hogwarts, the first place he has ever had friends or happiness, to protect the wizarding community. He follows his belief that danger is imminent, even when some adults disagree with him. He is willing to go alone, even if few believe him or want to help him. However, he eventually accepts the help of his two best friends, Ron and Hermione, when they insist on joining him. In the end, it turns out that his worries were realistic, and Harry plays a significant role in protecting the sorcerer's stone.