Which of the 3 schools is best for testing out of?

[TrailRunr]

Well, as I said, I could not personally do more than 1 SL course in a month, so that won't help me much.

You are missing out on some really, really low hanging fruit. You don't need to be gifted nor do you need copious spare time to finish 1 SL course in a few days, unless it's one of the exceptions like economics. All you need is control-F with your e-textbook during exams, which is permissible.