"As long as we're both in mermaid form we can't leave and we can't get help" says Amy. "We better dry off, get back to human form, and go from there."
You crawl across the locker room and find some clean gym towels. You and Amy each take one, trying to dry off. Slowly Amy's tail fades back to legs and her penis withdrawls back into a vagina. Meanwhile, you're still having fun with your female body as you dry off. You can feel another orgasm building and lay on your back on the floor. Your back starts to arch uncontrollably as you climax...and out pops your penis. You blink in surprise and realize you're not just human again, but male again too.
"This is so weird" you say. "I guess what I turn into on any given day is going to be based on what you're in the mood for?"
"I guess so." Amy gets a mischievous grin. "Should be lots of fun. Maybe I'll get more ideas..."
Then another voice interrupts. "What...what's going on?"
It's the girl who slipped and hit her head. Good news is she's not dying of a cerebral hemorrhage. Bad news is you had forgotten about her while drying off. How do you explain this?
"Hi. You look familiar" says Amy. "Lois? No...Lana?" She nods. "Uh...how much did you see?"
"You...you were both mermaids. And you had a dick. But now you're both human..." She looks at you. "And you're a guy? I've seen you two together around school. But before you...and now... I don't understand..."
"Just say no to genies" you say.
"It's complicated" says Amy. "And I think you've got enough of a headache already from that fall. Are you okay?"
"I'm still a little dizzy."
"We can wait here until you think you can walk and we can help you to the school nurse" you offer.
"And we can trust you to keep what you saw to yourself, right?"
Despite the splitting headache, Lana can't help from being turned on. She got to see two mermaids having sex. Now these two are sitting her naked in front of her.
"It'll be our secret" says Lana. "On one condition."
"Sure, anything" says Amy.
Lana licks her lips. "I want a threesome later with the two of you." After a pause she adds "Preferably as mermaids."