After picking yourself up, you examine the costume in your hands. It looks like a school uniform. A girl's school uniform.
You try to put it back on its hanger, but you're having trouble slipping the collar of the blouse back on. You're still a little frazzled by the fall you took, and that strange shock.
You don't want to be seen fiddling with a girl's outfit, so you quickly take it and the hanger it came on and duck into the changing room area. You think you saw the store clerk glance in your direction just before you entered the little hallway.
You head to the farthest room and lock the door. You proceed to unbutton the blouse so that the collar is wide enough to easily insert the hanger.
You undo the first two buttons before you notice something peculiar about your hands. They look thinner, and their fingernails are just a little bit more grown out than they should be. They almost look like a woman's hands.
Wondering if you're just imagining things, you look into the full-length mirror and gasp in surprise. Your face has changed in subtle but distinguishable ways. Your skin is softer, your nose is smaller, and your eyebrows and lips are thinner. Every trace of facial hair is gone. The hair on your head has grown out about an inch. It's still close to the crop you originally had, but now it's more tousled, softer and thicker. You look like a more feminine version of yourself.
Before the shock of this inexplicable change can subside, you feel a strange sensation. It's as if you are being deflated. The changing room slowly expands around you, and your reflection confirms that you are getting smaller. Your clothes become baggy to the point where your shorts and boxers slip right off your waist and fall around your ankles.
Stranger still is the sudden weight you feel on your chest. It's hard to tell from looking at the baggy folds of your once perfectly-fitting shirt, but you have an uncomfortable feeling about where and how this new weight is forming. You tentatively lift your shirt and stare wide-eyed at two perfectly round breasts. They are slowly expanding like firm water balloons, causing you to adjust your posture to stay balanced. Without shorts on, it's also plain to see that your hips are growing outward slightly.
Throughout this process, you watch in mystified silence. The only moment when you feel anything resembling fear or terror is right before your transformation completes itself. With the strangest sensation yet, your manhood shrinks in on itself. You stare in disbelief as it disappears between your legs. You bend over for a closer look and gasp as you behold the slit of a clean-shaven woman's vagina. Your vagina.
"No way," you whisper to yourself in a feminine voice. Somehow you've been changed completely into a girl. You throw off all of your clothing and stare at your naked body. You're very scared at the sudden and impossible change, yet you can't help but feel a little bit turned on by the sight of yourself. Gone is the 6'2" college boy. In his place is a smaller, but much prettier, girl of high school age. You realize she looks just about the right size to fit into that uniform.
The prospect of donning a girl's uniform turns you on just a little more, and you feel for the first time the sensations of your new womanhood. You shiver a little. Are you actually liking this? You have to admit that it's fascinating, in a freakish sort of way. You have a chance to feel what it's like to be a girl. This sort of thing should be impossible, but it's not. And if changing into a girl was possible, then changing back to your old self should also be possible, right?
The clerk would know. You're about to open the door when you realize that you're still completely naked. You glance down at your clothes, now scattered around the floor. They obviously don't fit you anymore. The only thing that would is the uniform. The clerk had told you each costume was "guaranteed to fit." You just didn't realize until now exactly what she meant.
You take a closer look at the costume. It's a typical three-piece high school uniform. The white blouse has long, puffy sleeves that end in proper cuffs. There is a knee-length skirt and matching vest, both made of black wool. There are no insignia or ties. You assume a buyer of this uniform would add their own accessories to the costume according to what school it would be from.
You finish unbuttoning the blouse. Undoing the skirt is a little more difficult, as the zipper is hidden, but you eventually find it by looking on the inside. Also, pinned to an inside hem of the skirt, there is a plastic pouch containing a bra, panties, and leggings. It's vacuum sealed, with a document that reads "ATTENTION: Opening this package demonstrates your commitment to purchase. All sales are final."
That makes sense, you think. The store's "Try before you buy" policy only goes so far. You're not sure if the garments are sealed for sanitary purposes or as a cruel joke on the store's part. You need to get dressed to go back out and talk to the clerk, but if you don't wear the underwear you'll be exposed where it matters most. The skirt doesn't look like it would even come down to your knees, increasing the risk even further.
You've got some options. Only one of them requires you to pay for the costume. A price sticker on the hanger confirms that you can afford it, but is it really worth the money if you're going to just wear it for a few minutes?