Months later, when Voldemort and his Death Eaters showed up at the school to take on Harry Potter, the students were given a choice. After Voldemort's announcement asking that they hand Potter over, Pansy Parkinson stood up in front of him, facing him. She looked terrified.
"Here he is! Here's Potter!" Harry's heart lurched at the thought of her wanting to hand him over but then she turned around, her wand at the ready. "If any of you want him, you have to come through me!"
A chorus of "and me" echoed through the Great Hall with students from all four Houses standing in front of Harry.
"Thank you, Miss Parkinson," said Professor McGonagall in a quiet voice. "Mr. Filch if you will please begin to escort out the younger students."
Slowly, the Great Hall began to empty out but there were many students from all four houses still there to fight.
As the battle is underway, DA members are running around casting jinxes and spells alongside members of the Order of the Phoenix. The Death Eaters were prepared for them. They were not prepared to see their own children and the friends of their children fighting against them. The Slytherin rallying cry as they entered into battle was "FOR CASSIUS!"
For the first time in a very long time, the four Houses of Hogwarts were united for the same thing. For freedom. For safety. For Cassius.