It took Jen a few minutes, but she eventually calmed down enough to examine her surroundings. Next to the bed she is sitting on is a table and under that table is a clear plastic container. In it is a bunch of different costume parts. The school nurse probably kept that for those who can't afford a costume this week.
In her desperation, Jen got an idea. If she really is becoming her costume, maybe she can control what she becomes. Maybe she can find some humanoid gloves to wear over her plastic hooves. And maybe some point elf ears to override her unicorn ears. And maybe a witch's hat to wear over her unicorn horn. And maybe some boots big enough to wear over the plastic hooves on her feet and some kind of dress to wear over her tail. Anything human to wear over her current costume.
She opened the box of costumes to access her options...