‘School supplies’ is secretly another word for 'cosplay supplies you’d never think of’. My favorite happens to be those super crappy plastic folders that are good for everything BUT holding paper!
Polyproplyne/poly plastic is the best for things in which you need a non-paper, colored source. They’re lightweight, thin, and cut easily, yet are durable and also look nice when light is shined through them. I used them for my Flan wings as well as for other things such as glowing Magnet headphones from Vocaloid, etc.
They’re durable for what they are but I would use them mostly to fill in gaps rather than make actual (large) structures out of them just because they’re flimsy in large pieces. They’re waterproof and the color doesn’t spill and they come in a variety of colors. However, due to their makeup, things such as spray paint doesn’t adhere to it and super glue is also iffy. Hot glue or some type of tape is what you need.