The woman leads you to a corner of the store where you find footwear of every imaginable size and type. You find a section full of black low-cut school shoes. She helps you measure your new shoe size and picks out the appropriate pair. You tighten the buckle on each shoe and stand up, looking yourself over in a mirror.
"Is it satisfactory?" she asks you.
"Yeah, I think so," you respond. You suddenly realize that you've been idly twisting your right heel back and forth while holding your hands behind your back in a positively girlish manner. You reset your posture quickly.
The woman seems to have noticed. "One side-effect of our costumes is that they impart some minor behavioral traits, to help you adjust to the role."
That explains a lot. But as you think more about it, you realize something serious. "Does that mean... I'm going to be attracted to boys?"
"No, it's nothing like that. The behavioral shift is perfectly harmless. You can dismiss it completely with a little concentration.
That seemed to take care of that. There was almost no downside to this little escapade. You'd be able to experience life as a girl for a little while, even go to the party as one, and you'd be able to change back and forth any time you wanted.
You imagine yourself showing up at the party dressed as a high school girl, and suddenly realize the flaw in this plan. Nobody would recognize you, or even believe you if you told them who you were. And if you just pretended that you were a girl, your uniform would hardly count as a costume.
You explain your intentions to the clerk, telling her that, while you're still interested in the girl's uniform, you think it's better if you stay a male and wear a different costume.
"You'll still have to buy this costume, though," she reminds you.
You acknowledge that, and tell her you like this costume and will probably wear it around town. You'd like a second costume for the party, however. "Do you have any recommendations?" you ask.
She rubs her chin for a minute. "As you can see, we have a wide variety of options. I'll go see if I can find something that would work best for you. In the meantime, feel free to browse around. Just don't leave the store without paying for that."
You wouldn't dream of it. As she walks off, you take another look at your cute girl self in the mirror. You don't quite look like party material, anyway. You need something jazzier. More interesting.
You begin combing the aisles, hoping that something will jump out at you.