He tells her he's a trans girl.

"I'm trans!"

"If that were so it would say so on your paperwork. We're an inclusive school, we accept women of every kind."

"I know that, but I'm not out to my family yet. My dad's very old fashioned, he wouldn't take it well. So my only choice was to keep it a secret and pretend to be my sister."

"Oh, oh, I see." She looked at him intently, trying to gauge if he was being honest, he tried to look his most sincere. When she still didn't look convinced he pulled out his trump card.

It was a last resort, and horribly humiliating to him, but it seemed the best way to convince her. He pulled down his panties and said, "That's why my penis is so small, I've been taking hormone replacement therapy in secret for years, and it kept me from growing, down there."

"Oh!" She said, looking directly at his tiny dick without any shame, "that makes so much sense! I've never seen one so small before, so I was wondering. Well, let's just say it's a good thing you're a girl. It would be a miserable life to have such a tiny dick if you were going to try to live as a man."

"Yeah, I can, uh, only imagine." His face burned while paradoxically his dick stiffened again. Why did his tiny penis love humiliation so much? He released the waistband of his panties again to hide his arousal.

The dean continued, "well, of course I apologize for my harshness earlier. Tell you what, I'll make you an appointment with the school doctor to make sure you HRTs are set and paid for by us. And I'll alert the school nurses and I suppose the PE teacher so you can change in private. From everyone else it will our secret, though I highly encourage you to tell your roommate ASAP, I think she has a right to know."

Joshua nodded and got out of there as quickly as he could, relieved he'd gotten out of such a sticky situation, but vaguely nervous about what would happen when he saw a doctor who thought he was a trans woman.