>>2338359>I dont want a girl that shrinks and grows.Don't worry, she just shrinks and is stuck that way forever.
>>2338360>I don't have any backstory as to why a girl would be roaming around school during a school day; I'm open to suggestions for that.[All ideas in this post are free for you to use without payment, credit, or compensation.]
A visiting professor brings some artifacts to school. While setting up the display before class, Girl figures out how to open one up, and releases the fairies of mischief inside. They "reward" her by shrinking her and making her unable to wear clothes. One fairy is kind, and tells her she can rid herself of the nudity curse if she captures the other fairies. But the fairies are all over the school causing chaos. Including possibly stripping other girls naked. There could be a sub-plot where she has to convinced the other stripped girls to help her, or at least not attack her.
>And if she's small, that would make it harder for others to see her. It's more embarrassing for a normal-sized girl to be seen naked in school.At 4 foot, she'd still need to be a 50 feet away not to be seen. And being shrunk is embarrassing even if you have your clothes. (The punch line at the end could be that she can wear clothes again, but doesn't get her height back.) The idea behind shrinking is to make the hallways not-hopeless.