Stacey is the kind of person that is a procrastinator, doesn't worry about exams and studying until like...right before. And then she tries to cram and panics a bit. She usually manages okay due to natural intelligence, but she could do better. Alas, no study habits.
Sylvie is kind of the opposite. She does study a fair amount - she's studied at least a couple hours every day, and so she feels prepared when she goes into exams and does well. However, she also doesn't study as much as she could - she'll feel that her upcoming OWL scores could have been a bit better.
Marietta studies hard and then still panics when exams come because she wants to make her mum proud. Aww. And yet, no matter how much she studies, she always has problems with wandwork and her scores are only mediocre. :( So I guess she has a reason for panicking.