Ron had provided the firework for him to throw, suggesting Harry throw it into Malfoy's cauldron and like a fool he'd done it. He could've refused. He could've just thrown it into the air and let it distract the classroom. He still would've gotten into trouble but it would've provided the distraction she claimed to need all the same. Yet he'd done what Ron suggested without hesitation. While he wouldn't say Ron knew he could have seriously injured or killed Malfoy by doing so, he did know Ron wouldn't have lost any sleep over it if that had occurred. Because Ronald Weasely didn't like Draco Malfoy. But also because Ron didn't actually throw the firecracker. Harry did. And just one more step in his brewing would've seen Draco severely harmed by the explosion that firecracker had caused. One more step.
She'd slipped from the room while the Professor was tearing well deserved strips out of Harry's hide. That meant, in the entire incident, he was the only one to actually do something wrong though at the time he'd thought they were all equally guilty of wrong doing. He really should have figured that out at the time since there was no way Snape would've missed Hermione entering his private storeroom. Naturally, he was the only one who actually got into trouble over the incident. But he hadn't caught on. Hadn't figured it out. Just like he'd never paused to wonder why exactly Ron had a firecracker from his brothers, the twins, supply of mischief makers in his pocket that day. If he could get a firecracker, couldn't he also have gotten something less volatile like an untainted candy or the wrapper from one? Neither of those would've caused an explosion that could hurt anyone. But then, an explosion is what Ron had wanted. And if someone got hurt, all the better for Dumbledore's plans. Because that was leverage to use against Harry.
He'd been a fool. He'd been happy to take the fall, thinking he was sheltering his friends and glad to do so since they were helping him prove his innocence. How they must have laughed at him. So foolish. So gullible. So stupid!Hindsight made it clear what he'd done had been extremely dangerous as well proving to Professor Snape that Harry was every bit his father's son as he had always claimed he was. And because Malfoy had actually suffered a real injury, minor but still real, he'd given Dumbledore and his cronies all they needed to further isolate Harry and set him up to his peers. Something Harry had never been able to recover from. Something he was still forced to deal with even now. Years later.
Harry had been removed from Gryffindor Tower and into Protective Custody by the Headmaster and his Head of House when it became clear none of his dorm mates were willing to stand up for him or wanted to share a room with him any more. After all, even though they all knew Malfoy had been getting in his face and taunting him ever since he'd first set foot into the magical world, he'd still hurt him. And if he could hurt Malfoy, who wasn't even in their Tower, just for taunting and teasing him, what could he do to them if they upset him? Thanks to Ronald talking about him behind his back, those other boys in his dorm all believed he had a superiority complex and a hair trigger temper. They also believed he was quite capable of holding grudges and getting payback against those he felt had somehow wronged him which Malfoy had done by bad-mouthing his parents and the manner of their deaths.
He was given his own chambers on a different floor with a portrait under strict orders not to let anyone except the teachers, Headmaster, house elves and Harry inside the room without express permission, to guard the entrance to his sanctuary. Because Dumbledore had provided the portrait with Harry's class schedule, the door portrait would go into lockdown as soon as Harry was escorted back after his last scheduled class. It wouldn't unlock until breakfast time the following morning except for one of the afore-mentioned persons minus Harry himself. In essence, he was a prisoner and there was nothing he could do about it but serve his open-ended sentence. All for throwing a firecracker into the cauldron of another student.
He had been heavily and obviously escorted to and from all his classes allegedly because it wasn't safe for him to be in the halls alone and since he was being shunned by his school mates, there was no one else to walk with him. Once a day, he'd be escorted to a bathing room by one of the male teachers, usually Professor Snape who spent the entire time regaling him with what a waste of time and space he was, so he could take a shower but otherwise he spent all his time in his room. He got used to the invasion of privacy as Snape refused to leave Harry alone in the shower room for his daily showers. How could he rag on Harry if he wasn't in the room with him? He wasn't even allowed to visit the library for extra books on homework assignments as the teachers claimed not to have the time to stand guard over him while he was there. So his grades, which were already only average, took a nosedive since he had no reference materials outside his own textbooks to use. And no matter how many times he asked, Hermione always seemed unable to bring him any extra books for his assignments. Either she forgot or the book was checked out or in use by another student. Or she was currently using it herself but she'd give it to him when she was done with it. A story Harry knew even then meant too late to use it for his homework.