Where were the other students during the Battle of the Astronomy Tower?

Question

In Harry's sixth year, Draco Malfoy let Death Eaters into Hogwarts through the Vanishing Cabinet. Teachers, the Order of the Phoenix, and Dumbledore's Army fought them throughout the castle.

Where were the rest of the students?

Answer 1

Its mentioned at the beginning of Chapter 25 of Book six that it was already June when the Death Eaters attacked:

But Ron's tolerance was not to be tested much as they moved into June, for Harry and Ginny's time together was becoming increasingly restricted. Ginny's O.W.L.s were approaching and she was therefore forced to revise for hours into the night.
_{Harry Potter and The Half-Blood Prince: Chapter 25}

When Dumbledore and Harry left the school and headed towards Hogsmeade, it was almost sunset time (twilight),

They made their way down the drive in the gathering twilight. The air was full of the smells of warm grass, lake water and wood smoke from Hagrid's cabin.
_{Harry Potter and The Half-Blood Prince: Chapter 25}

In addition, they were gone for a couple of hours at the very least. Using all this information along with the [apparent] location of Hogwarts, it could have been well around midnight when the attack happened. So all the students would be in their dormitories, sleeping.

One could argue that some students might be out of bed. Yes there were. The members of the DA were patrolling the corridors with Hermione and Ron as lookouts for any suspicious incidents.

Remember, Voldemort's return had been known to everyone by the end of Book 5: Order of the Phoenix. The school already had several additional security measures since the beginning of the year. In addition after-hours wandering was also not allowed.

Now, as everybody in this Hall knows, Lord Voldemort and his followers are once more at large and gaining in strength."

The silence seemed to tauten and strain as Dumbledore spoke. Harry glanced at Malfoy. Malfoy was not looking at Dumbledore, but making his fork hover in midair with his wand, as though he found the headmaster's words unworthy of his attention.

"I cannot emphasize strongly enough how dangerous the present situation is, and how much care each of us at Hogwarts must take to ensure that we remain safe. The castle's magical fortifications have been strengthened over the summer, we are protected in new and more powerful ways, but we must still guard scrupulously against carelessness on the part of any student or member of staff. I urge you, therefore, to abide by any security restrictions that you teachers might impose upon you, however irksome you might find them - in particular, the rule that you are not to be out of after hours. I implore you, should you notice anything strange or suspicious within or outside the castle, to report it to a member of staff immediately. I trust you to conduct yourselves, always, with the utmost regard for your own and others' safety."
_{Harry Potter and The Half-Blood Prince: Chapter 8}

After all this as well, any rule-breaking out-of-bed students would be protected by additional security measures the members of the Order of the Phoenix.

'I ... they're up to something!' said Harry and his hands curled into fists as he said it. 'Professor Trelawney was just in the Room of Requirement, trying to hide her sherry bottles, and she heard Malfoy whooping, celebrating! He's trying to mend something dangerous in there and if you ask me he's fixed it at last and you're about to just walk out of school * without -'

'Enough,' said Dumbledore. He said it quite calmly, and yet Harry fell silent at once; he knew that he had finally crossed some invisible line. 'Do you think that I have once left the school unprotected during my absences this year? I have not. Tonight, when I leave, there will again be additional protection in place. Please do not suggest that I do not take the safety of my students seriously, Harry.'
_{Harry Potter and The Half-Blood Prince: Chapter 25}

---

So,

• Late hours and supposed curfew ensured that the students were in their dorms when the attacks happened.
• Members of OoTP ensured the safety of any students wandering students (if any).