This was good and please write more! I LOVE NEVILLE! Sorry, that was a bit random but not completely off-topic.
In this section: 'That night, during dinner, I sat with Ginny, Dean, and Seamus. They had heard about the "demonstration" that Carrow had done in our class; oddly enough, he didn't do it in any of the others.', I would have thought that Dean and Seamus would have been in the same class as Neville, because they are in the same year and the same house.
I enjoyed reading this. An interesting concept for a story, I have been looking forward to reading a fiction set around Neville's 7th year and you have portrayed him well. Is it a chaptered fic? I hope it is, I can't wait till the next chapter.
You have some good things going on in your story, but you also have some things that need work. The biggest things to work on are descriptions and characterization.
This is in the first paragraph, and the descripion is a bit distracting:
My heart was falling a million miles per second into the earth, as I went looking for a compartment to wait for Hogwarts to arrive in.
First, everything before the comma is nice, but it feels like you’re trying to sound poetic instead of just writing. It’s almost like you added this sentence in to be creative, but it doesn’t really flow with the rest of the story. As for everything after the comma, “I went looking to wait for Hogwarts to arrive in.” It just doesn’t make sense. You don’t wait for Hogwarts to arrive. You wait to arrive at Hogwarts. Some of the phrases you’re using just make that sentence confusing. You might want to read your sentences a bit more carefully, just to make sure everything you wrote is what you meant to say and that it’s clear.
This next example is one of characterization. Neville:
"S-so what do we do?" I stammered, frightened at the prospect of me having to say it. "Snape's headmaster now. That probably means that some big changes are on their way. We can't wait for them to arrive, carrying our salvation for us."
You try and portray him as stammering and awkward, but in the same paragraph, he’s suddenly the leader and confident and making decisions to lead a group of students in a rebel force against Hogwarts before school even begins? It’s possible, but not probable at all that something like this would happen. It’s hard to have your character portrayed two different ways, especially at the beginning of your story, and have your readers believe what you’re writing.
"Yes!" Ginny and Luna both hastily agreed.
This is also awkward. Suddenly Ginny and Luna are following Neville? They’re instantly on the same team and hastily starting a rebel force before they get to school. I understand Ginny’s bitterness—that’s something you hit spot on in your characterization—but I don’t understand this immediate agreement on the plan. I know it’s something that has to happen, but I just don’t see anyone but Ginny feisty enough to start something on the train.
I’m a bit confused on your Luna. I don’t remember the Luna from canon being a giggler, but you have her giggling every time she speaks. It seems a bit out of character for her, and it’s distracting to read.
"Welcome, wizards- and witches-in-training," he started, his voice as sour and dead as usual.
I would never describe Snape’s voice as “dead.” I would describe Professor Binn’s voice as dead. Maybe I would describe an exhausted person’s life as dead. But I’m not sure Snape could ever speak in a voice that sounded dead—especially not when he’s in a position of power in front of his students.
Then in DADA, I’m confused at the lack of reactions to Professor Carrow’s question and assignment of the Unforgiveable Curses. Don’t any of the students remember their fourth year when the Death Eater in disguise, Professor Moody taught them a similar curriculum? Or are they just all silent. I’d think that Neville, in his thoughts at least, would be replying/responding more to Carrow’s questions than thinking that Hermione would know if she were there. She did know, but so did Neville in fourth year. And all of them should have known because they all learned it then.
Also, I’m not sure a foot “nudging” Neville is enough to make sure he’s okay. And you never mentioned him being near a wall at all when he was getting the curse. I was startled to even see that he had a scar so immediately. Scars don’t happen that fast, he would at least have had a wound of some sort at dinner.
The best part about your story is this line: They were here to destroy Hogwarts; we were here to save it. I believe that captures the essence of what you’re after. If you could make this the focus of your story, and everything that happens, I think your story would connect more. So although you have a lot of characterization issues, I think you story has potential because of that last significant line: students saving the school. It’s interesting… good luck in making it happen!
I LOVE IT!!!!!!!!!!!!!!!!!!!!!!!!! YAY!!!!!!!!!!!!!!!!!!!!!!!!!! Go Neville... woot woot!