He admitted to following Ron into the rooms under the Whomping Willow to prevent injury from befalling either of his two friends. And he admitted to forcing Ron to allow Professor Lupin and Sirius to use the anti-animagus spell on the rat which had prompted a lecture on respecting and protecting the belongings of others wherever and whenever he had the opportunity to do so. That his intention had been to settle the matter once and for all was lost in the ensuing lecture about property ownership and the duty of friendship. He admitted to hitting Professor Snape with an Expelliarmus spell to disarm him though he insisted he hadn't meant to hurt or knock him out. All he wanted to do was lower the animosity in the room and get the Professor to take in the full situation since he clearly hadn't yet realized there was a supposedly dead man standing there very much alive in the room. Then the Professor could make the decisions.
Madam Pomfrey made the Ministry representative and Dumbledore both leave him alone after twenty minutes of their constant and repetitive questioning. She force fed him a very powerful sleep aid that, unknown to Harry, Severus made specifically for her to give him. She knew well of his trouble with sleeping and his built-up resistance to most the sleep aids on the market. So she had enlisted Severus to make her a better aid for him. Nor had she missed the dead quality of Harry's voice as he told the story from his point of view. A story that hadn't wavered a bit no matter what his interrogators said to try and force it to change. The child needed rest and she was determined to see that he got some.
Snape had raised a brow when he saw which potion she intended to give Potter but she had refused to satisfy his curiosity so long as the others were in the room. Like everyone else in the school, she was well aware of his dislike of everything Potter and how anything the man could hold against him, he gladly taunted the boy with repeatedly. And yet, she was also aware of just how many times Snape had saved the boy's life and fought with Dumbledore over the risks he encouraged the child to take with his personal safety.
Morning light brought the news that all three of the Marauders were dead or as good as dead now. Supposedly their bodies were found tangled together on the shores of Black Lake where apparently they had stood together one last time to fight a common foe but that foe had apparently been stronger than they were. But Harry knew for a fact Remus Lupin was not dead. He had after all, heard him reporting the events of the confrontation between Harry and Sirius. And he couldn't have done that if he had died on the shores of the lake. Not unless he'd come back as a ghost. Which unlike Colin who often kept Harry company in his room when no one was speaking to him, he hadn't.
At the time Harry had believed both Sirius and Peter most likely were dead simply because they had become a liability to Dumbledore now and he had no proof to the contrary. Later he would discover Peter wasn't dead any more than Lupin was. And that would make him wonder about Sirius. After all, if Dumbledore was the source of the rumor, which in this case for Harry he was, it was almost guaranteed to be at least partially untrue. He'd come into the infirmary to personally give Harry the news that his Godfather whom he'd just met the night before was as good as dead along with his two friends, Remus Lupin and Peter Pettigrew. He spun some tripe about how all the Marauders were now together again in Heaven where they'd be watching over him hoping he'd do the right thing when he came to times in his life where choices had to be made and take his rightful blame when he made the wrong choices.
According to school rumor Peter's form showed clear damage from long jagged teeth and nails while Sirius had been given the Kiss by the dementors. The official story was both Remus Lupin and Sirius Black had administered their own justice to Peter for his betrayal of them all those years ago. Sirius' body showed slight bruising and a few shallow cuts though overall he had relatively little clear cut damage suggesting that while Peter got a few licks in he hadn't been able to hurt Sirius enough to effect his freedom. It also suggested the cause of his death was the dementors that were suspiciously missing from their posts around the Castle Grounds.
Harry was allowed to recover in the infirmary and deal with the new deaths away from prying eyes and stupid questions for two days before he was summarily teleported back to Privet Drive early as punishment for his attack against Professor Snape. He wasn't expelled but he had been suspended. And even Harry knew that was because he wouldn't change his story to accept all the blame for each little wrongdoing of the night. Hermione and Ronald had the remaining two weeks of school to make sure everyone knew the story of the night though and how Harry Potter had figured into the mess. No one would ever know he hadn't stayed locked down in the infirmary for those two days. Not even Madam Pomfrey knew he had slipped out to go spy on the old man. Or go steal back his map.