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.
We’re talking about a confrontation between a Jedi Master and his apprentice, not between a chemistry teacher and a student. Of course Ben would know Luke can sense his feelings and vice versa. Therefore lying about it wouldn’t help Ben in any way. The difference is, that this way would be honorable and out in the open, with Ben aware of the situation. However, like I said, since the film didn’t bother to give us some context, it’s all speculation, and all we have is a fearful Luke sneaking into Ben’s room, rather than face the issue head on, for some unknown reason.