Tell the truth.

Panic did take grip of his head, but Alexander remembered that he hadn't done this out of pervertedness. As far as he was concerned he hadn't done anything wrong at all, so if he could tell her exactly how he convinced himself to do it, maybe she could be convinced that this was what he should have done.

"Uh, yeah... funny story..." Alexander shot Hailey an innocent smile, "I ran out of hot water, so... I came here instead..."
Hailey stared at the boy nonplussed with folded arms.
"...because... I needed a hot shower... and I didn't think anyone was here..." Alexander cleared his throat as he realized this didn't sound as persuasive as he had hoped, "...sorry."

"Yeah, well I am here now, and from now I have to start worry that boys are running around here all the time whenever stuff like this happens."
"Yeah, well... it's not like I wanted to either, and like I said... I'm really sorry. Besides, shouldn't you be working now? What are you doing here?"
"That's none of your business." Hailey said, "The point is that you screwed up bad... real bad."
"...so you're going to report me?" Alexander realized a serious infraction like this could cost him dearly. It was likely he could kiss his chance of promotion goodbye, and he might have to spend time in the dungeon for this.
"...let's not get carried away here. See, I don't think you're actually dangerous, so what's the point of having the other girls worry about what happened here?"
Alexander felt his heart skip a beat as he saw a sliver of hope, "So you're letting me go?"
"Letting you go?" Hailey smiled deviously as she looked up and down Alexander's lean, handsome, naked body, "Those aren't exactly the words I'd use, but I won't report you. However, you'll owe me one. You'll owe me big for this. I'll let you finish up here, and then meet me at my room in twenty minutes. Now chop chop! You don't wanna be here when the other girls arrive!"
Alexander's ears drooped. He didn't care for this development at all, but what else was he supposed to do?
"Yes... Hailey. Whatever you say." he said meekly and quietly.
"Oh, call me Mistress Hailey from now on." Hailey said with a satisfied smile.

"...all right, Mistress Hailey." Alexander said even quieter.
This had turned out to be a very long evening, and it was far from over.