Did Professor McGonagall order that all Slytherin students leave when evacuating for the Battle of Hogwarts?

Question

There's a scene in Harry Potter and the Deathly Hallows chapter 31 where Professor McGonagall's words seem ambiguous to me. The scene is when most of the students are sent through a secret passage to leave Hogwarts before the battle. Students that are 17 years of age or older can choose to stay. Pansy Parkinson proposes to give up Harry Potter to the Dark Lord.

‘Thank you, Miss Parkinson,’ said Professor McGonagall in a clipped voice. ‘You will leave the Hall first with Mr Filch. If the rest of your house could follow.’

Harry heard the grinding of benches and then the sound of the Slytherins trooping out on the other side of the Hall.

‘Ravenclaws, follow on!’ cried Professor McGonagall.

Slowly, the four tables emptied. The Slytherin table was completely deserted, but a number of older Ravenclaws remained seated while their fellows filed out: even more Hufflepuffs stayed behind, and half of Gryffindor remained in their seats, necessiating Professor McGonagall's descent from the teachers' platform to chivvy the under-age on their way.

Did this sentence mean that Professor McGonagall ordered all the Slytherin students to leave, even the ones of age? Or did they all just decide to leave on their own? (I'm asking only about the students. I know that Professor Slughorn, the Head of House, remained.)

Answer 1

Looking at the quote in question, it's important to notice the next part.

Harry heard the grinding of benches and then the sound of the Slytherins trooping out on the other side of the Hall.

‘Ravenclaws, follow on!’ cried Professor McGonagall.

The Slytherins were not forced to leave, they just all choose to leave. We see that in this sentence:

The Slytherin table was completely deserted, but a number of older Ravenclaws remained seated while their fellows filed out: even more Hufflepuffs stayed behind, and half of Gryffindor remained in their seats

The observation that the Slytherin table was completely deserted is pointless if McGonagall kicked them all out; it is, however, noteworthy if none of them stayed, compared to the rest of the houses.

Update, this quote clearly shows McGonagall's intention to offer everyone the chance to stay

‘The time has come for Slytherin House to decide upon its loyalties,’ interrupted Professor McGonagall. ‘Go and wake your students, Horace.

Ernie Macmillan stood up at the Hufflepuff table and shouted, ‘And what if we want to stay and fight?’ There was a smattering of applause. ‘If you are of age, you may stay,’ said Professor McGonagall.

And her curt remark in the quote in question is due to Pasny Parkinsons comment here,

Then a figure rose from the Slytherin table and he recognised Pansy Parkinson as she raised a shaking arm and screamed, ‘But he’s there! Potter’s there! Someone grab him!’

Before Harry could speak, there was a massive movement. The Gryffindors in front of him had risen and stood facing, not Harry, but the Slytherins. Then the Hufflepuffs stood, and, almost at the same moment, the Ravenclaws, all of them with their backs to Harry, all of them looking towards Pansy instead, and Harry, awe- struck and overwhelmed, saw wands emerging everywhere, pulled from beneath cloaks and from under sleeves.

Its very clear that McGonagall gave the Slytherins the chance to join the fight, and they chose as a whole to leave.

Answer 2

To a degree

The quote given in the question makes it very clear that McGonagall asked the Slytherins to leave. However, this was not extraordinary: McGonagall asked all the houses to leave, hoping to keep most students safe. She requested that Ravenclaw leave right after telling Slytherin to do so. That said, the fact that she asked Slytherin first was probably due to her dislike of the house.

It is important to understand that McGonagall did not intend to force Slytherin to leave, if they wished to stay and fight. It was merely a strong suggestion, and thus did not really rise to the level of an order. In fact, much of Slytherin house did not leave due to mere cowardice or due to their sympathies toward Voldemort, but saw themselves as leaving a hopeless fight to go get reinforcements.

JKR: A part of the final battle that made me smile was Slughorn galloping back with Slytherins, (SU: Yes!) (JN laughs) but they’d gone off to get reinforcements first, you know what I’m saying? But yes, they came back, they came back to fight, so I mean- but I’m sure that many people would say “Well, that’s common sense, isn’t it? Isn’t that smart, to get out, get more people and come back with them?”