"…you will grow two extra heads.
"Not a human head, of course, that's be ugly. To set things up, you will immediately become an anthropomorphic whippet, and will grow two additional dog heads to the side of yours, akin to the mythical Cerberus. You retain conscious control of your body except your bladder, bowels, and any head other than your own in the middle. Instead, the heads are inhabited by independent consciousnesses of normal dogs and will behave instinctively like dogs, sniffing, barking, whining and whatnot, unable to speak. Your own head will still become that of a dog but will not be subject to canine instincts and will retain your present mind and the ability to talk, though you can bark through your muzzle if you wish."
Fur grew all over her body at the same time, creamy white overall with a few brown spots. She tried to caress her own belly to feel the fur on herself only to find her fingertips feeling stranger by the second, with claws growing from the end and pawpads in the palm and on each finger, forming a paw that could still grasp like a human hand. Her arms and feet slowly became much more slender while her feet shifted to force her to stand on tiptoes, the pinky toe shrinking into a dewclaw while the other toes develop claws and pads too. A slender, slick tail extended from her rump, her hands inadvertently trying to caress it gently.
The most drastic change had yet to come. Above her neck, her nose and mouth began to stretch forward into a canine muzzle, her ears migrating to the top of her head and turning floppy, her eyes turning all black with the vision becoming duller, while the same fur that covered her body sprouted there too. That's not all, as she felt a lump grow from each shoulder, slowly taking shape of canine heads like her own. Just as they fully formed, Jennifer lost control of them, as they spun around and sniffed on their own in confusion, trying to take stock and make sense of the world they spontaneously emerged in.
She ran over to the mirror and noted her nice slender whippet body in the mirror. Damn, that looks like so many pounds lost… until the heads beside her began viciously barking at their reflection(s?), even threatening to pounce at the mirror. Unable to stand the noise, she walked away from the mirror and back to her phone, the other heads eventually shutting up.
"Once again, every time you lie, this process of growing new heads on each side of your shoulder will repeat, with all of them being whippet heads. For your dare, you will relieve yourself at the whims of a random head each time you feel the urge, but never your own. Have fun at school soiling yourself!"
As he hung up, she already felt the urge to go pee. Even if she ran to the toilet at full speed, her left head began to sniff around, and once it found the spot, stick out its tongue as the pee tricked down onto the carpet, before she could reach the bathroom. She raised her leg to aim at the wall instead, minimizing damage even if she couldn't hold herself in. Once she was done, the heads on each side turned towards the patch of wall where she had left her mark, trying to sniff it. Even if she wasn't controlled by instinct, curiousity got the better of her and she explored the patch with her newfound nose along with the other heads. So this is how dogs recognize their own territory…