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.)