How does this work for transfer students? Do we-or rather, do I, since I think I'm the only sixth year one-post my CATS scores and class choices the same way?
Though I object on the grounds that its an invasion of my privacy that everyone should know my scores.
Furthermore, if someone never had a subject at Salem because it was never offered, are they eligible to take it here?
Scores up, just like everyone else *grins* You've got to pick classes, so the Professors need to know what they're dealing with and whether they'd take you on.
As for classes you didn't take at Salem because they weren't available, that would be... Muggle Studies, Ancient Runes and History of Magic, if I recall everything correctly. Ancient Runes you'd definitely need the background in learning about it. The other two, you can ask the Professors if they think they would take on a complete beginner into their Advanced Classes.
Perhaps make up classes? Either way, it's up to them and to whether you're interested enough in their subjects to take on the extra workload.