Given that you don’t need to be an IB student at all, there’s really no such thing as a minimum score. The important thing is that you’ve taken courses at that level and done well in them; doing well on the exam is just confirmation that you know the material. A high enough score is possibly an opportunity to get out of intro-level courses, though; you’ll need to check the school’s website to see how well you need to do to qualify for that. You can also find a rundown of the average SAT range there, though I’m guessing it’ll be in the very high 600s or above for each section.