The usual term when I was in school was scratch paper—scraps of paper you "scratch out" calculations and notes on, to be discarded after results have been fair-copied into your exam answer or essay.
- Laure calls my attention to scrap paper, which appears from this GoogleNGram almost always to have been more popular in BrE, and to have become more popular in AmE since I left high school.
- Jim calls my attention to the alternative term work sheet. This works, too; but in my experience it is usually employed when you are supposed to turn in the paper, with your exam, rather than discarding it.
A working paper is usually either a) a report on a "work-in-progress" or b) a "working" version of a paper which is still in development and not yet ready for submission or publication.
The traditional term in textual criticism for an author's manuscript draft, often full of strikethroughs, insertions and corrections, is foul papers, from which a "clean" version or fair copy is made.