What were the Slytherins doing during the battle of Hogwarts?

Thank Quora for this answer:

Let’s tally up the Slytherins whose whereabouts we know…

When it became clear that the Slytherins were quite willing to deliver Harry to Voldemort, Pansy Parkinson and the bulk of them were escorted out of the Great Hall first, to be brought to the Room of Requirement and then through the Hog’s Head, where the students were expected to escape from the upcoming battle. If we go by Voldemort’s words:

“He (Draco) did not come and join me, like the rest of the Slytherins”

We can assume that, instead of going home, they instead joined Voldemort’s forces. Likely most if not all of their parents were either Death Eaters or Voldemort followers.

Crabbe, Goyle and Malfoy had doubled back a lot earlier than that, and decided to lay in ambush, under a Disillusionment Charm, by the door of the Requirement Room, hoping to capture Harry so they could deliver him to Voldemort. They may not have been fighting by his side, but they were still fighting ON his side.

So it would appear that each and every Slytherin student fought for Voldemort bar none, and the only Slytherins who fought against Voldemort were the teachers - Slughorn, openly, and Snape, covertly.