When should i approach?

[Bugsy Ant]

There's a chick that i want to ask out/get her number etc who i have spoken with before i know where one of her classes are but i dont know whether i should ask before class or after. She sometimes comes late and i don't want to make it seem like i was following her or waiting(or have her think that), which i doubt she will think it's ''cute'' What should i do???