Another option for a highly motivated student who has been barred from an AP class that they really DO have the ability to succeed in, is to go to Modernstates.org and take the class online for free, and just take the AP exam, without having taken the AP class in the high school. It’s got to look pretty impressive if you can get a 4 or a 5 on an AP without ever having taken the class. It could even be used as a hook for an admission essay, about how your persistence as a motivated, self-directed learner allowed you to overcome the lack of available seats in AP classes at your high school.