"She prepares to start school."
Translation:Hon förbereder sig för att börja skolan.
Is it skolan instead of skola because you're talking about a specific school and not school in general?
No, börja skolan means just to start school. It sounds weird to not have it in the definite.
You use the preposition för with förbereda. You say förbereda sig för något.
On multiple choice I was marked wrong for not choosing "forbereda sig pa" ?
That's also an option, förbereda sig på is used pretty often and the meaning is more or less the same. However, we want to minimize the things showing up in multiple choice questions that can be tricky, so I'll try to remove it as something that can show up there.
No, it sounds strange. The verb starta is used with things you control, that don't have agency. You would starta en motor, but things like school or a theatre play are considered to be more like things happening to you, and require börja instead.
förbereda is effectively transitive in Swedish, meaning that you need to prepare something specific. If it's yourself, you specify that through the reflexive pronoun.