You can't decide, but it will be fun. She's standing to one side, still stretching. You get to see live as she stops, staring at her hands. She starts feeling herself down, getting a few weird looks from teachers. She turns her back to the students as she looks herself over.
"Ms. Janett's butt's really sensitive. Hitting it is a major turn on for her. She loves whenever anything touches her butt." You would've done tits, but she turned away from you. She's also standing right next to the circuit, a rookie's mistake.
You go to the outside of the ring, accidentally running into her and her butt. It isn't a head on collision, but you brush past and your arm feels something under those running sweat pants. Something bigger than you thought.
As you "accidentally" touch Ms. Janett's apple bottom, she wriggles with a small moan. Her arm immediately comes up and covers her mouth, blushing immensely. She speaks up, stuttering slightly. She even sounds younger, still stone cold though like she's mad at something. "You all need to run forty laps total, I'll watch from my office. When you're done get changed."
You now pay a little more attention to her bum, not huge but strangely soft. Like most of her, it's hard to see with her track suit in the way. The other students are excited to work less while she goes into her office, then counter productively shuts the blinds.
You sneak over, since you can do whatever you want, peeking in. She's trying to sit down, but moans every time anything touches her butt, even the seat. She can't seem to stop squirming. Eventually, panting, she gives up on chairs, leaning over the desk and looking inside her clothes. You could change her here, but that's not very embarrassing.
Eventually her breathing calms, and she stands up and takes a step back before letting out an effeminate "Eep!"
The mere rub of her own pants is making her horny. She slowly walks, moaning as tries to get control. You hop back to the other students position, smiling as she trudges out of her office. She scans the remaining running students, closing the door behind her. You wait until she gets away from the office so she can't retreat, then...