Casual Academics

Can you apply to become a casual academic for a course that you're doing now?

For example, if I'm taking a course in 24T2, can I apply to become a casual academic for a 24T3 course?
Just a bit confused because they said they'll be filling in positions before the term ends - then how would they know if I'll even pass the course? (I've seen people fail a course because they didn't meet final exam requirements despite doing really well for pre-exam assignments/quizzes).