How would the school react to a terrorist attack from a Muggleborn upon the dorms of House Slytherin ?

Let's imagine a seventh year student. A muggleborn who is marginalised, bullied by slytherins of his year, and dable in the Dark Arts in his free time. After hearing the famous "you will be next, mubloods !" quote at Haloween, he decide that House slytherin is rotten and shall be purged. He ambush then crucio a first year slytherin to get the password to enter the common Room of slytherin, then lay waste to the first students he sees with the Killing curse. He end up stunned, but not before taking the lives of 5 students. What happens next ?