I'm going to start looking through journals and such this week! Thanks for the feedback! Any books or online articles you happen to come across about enrollment rates would be greatly appreciated!
OK, so the next thing you need to learn, since this is a Senior Thesis, is to how to access, search, and use the databases that will direct you to articles on admission rates studies. Talk to your university library staff. (Few scholars "look through journals" except for general keeping-up-with-the-field purposes; when looking for material on a specific topic, or on a broad topic that needs to be narrowed into a specific topic, we begin with the subscription databases in appropriate fields that are held by our libraries.)