Trick her into playing along

"So, there's another complication..." Nikki started, distracting Kiyomi from how she was removing her useless top, to eliminate any chance of covering up.

"I had to make up a story to keep you out of trouble... see, the nurse wanted a solid reason for why you were dressed like that. Because it kinda is grounds for expulsion... so I told her you were an exhibitionist! I warned her you would play innocent in front of the camera, but otherwise and in the future, you'll have to try to play along. Don't worry, it'll all work out!"

Kiyomi couldn't fathom what this meant, but Nikki had some idea about how this could go. Nurse Lee might bring it up with the principal; they might try to accomodate her exhibitionist ways in some kind. Make her seek an outlet in class activities. Maybe they'd even notify Nikki's parents, and recommend helping her live out her tendencies! And as long as Kiyomi believed she needed to play along to avoid expulsion, there was no stopping it.

Kiyomi couldn't do anything than grunt in agreement, as she felt the boy holding open her downstairs change his grip.

"How are you feeling, Kiyomi? You holding out? The nurse will come back in a second..."

"I... N-Nikki, I'm really... I might... cum..."

Her sister caressed her exposed chest, eliciting a moan.

"That's okay, it's okay to feel good. And look, there's the nurse! You will feel *great* in a moment, I promise."