Ben? Are you falling to the Dark Side?
No, Master Skywalker!
Good! Carry on with your exercises then!
Ben had already fallen to the Dark Side, and probably already swayed the students that left with him. (Presumably they became the Knights of Ren?) His reaction to Luke wasn’t feeling betrayed by his master, it was being found out. Luke’s school was doomed whether he went in Ben’s room that night or not.