I went back and look at this part of the book. The Slytherins weren't given a chance to stay and fight. It's at the beginning of chapter 31.

The entire table left, with Filch, because McGonagall ordered them
out, "thank you, Miss Parkinson. You will leave the hall first with
Mr Filch. If the rest of your House would follow."

As Mcgongal said this the rest of the student body was pointing wands at
the Slytherin students.

I sure if they weren't at wand point some would have stayed to fight. I was surprised that Slughorn stayed behind though - he seemed like a duck and cover type of character to me.