Bella goes off to the library.
She looks up memory charms (defenses against).
The material on Occlumency doesn't seem to have anything about Memory Charms at all.
It is a rare and difficult skill, and not commonly taught at wizarding educational institutions.
Bella wants to learn it. Memory Charms are bad, Legilmency is also bad. This is her top priority. Only the fact that she doesn't suspect anyone of actively being interested in the contents of her mind will allow her to do homework or anything before she has this mastered. She wants introductory exercises and ways to check her progress that don't involve inviting somebody to point a wand at her; can she have them?
Well, what about just the exercises, then, maybe she doesn't have to benchmark her progress, maybe she can just work like her brain depends on it and get as far as she can -
Bella goes up to the librarian. "Excuse me," she says. "Do you know if there's any way to get lessons in Occlumency?"
"It's defensive," says Bella, puzzled, holding up one of the books she found. "How is that a dark art?"
"I really want to learn it. Is there a correspondence course or anybody on the faculty who knows about it? The books aren't - aren't instructional," pleads Bella.
"Thank you," says Bella. "Do you need my name so you can tell me if you find something?"
"I'm Bella - Isabella - Swan, and I'm in Analisa Hall room four," Bella says. "Thank you so much!"