i've exchanged spells with other witches in my school while in class and looked up different witch sites on school computers. in terms of casting, once, maybe twice. i brought a clear quartz crystal i had used in a good grades spell to my exam once, and i think i cursed someone who wronged me in a big way. the curse is just a chant, one of the first i learned actually, just takes focus.
simple spells like those, chants and luck charms, are fine to cast at school because you're not drawing attention to yourself really, and they're typically cast to help you when there's a pop quiz, or you want time to go faster.