It would be in your best interest to try and get all of your pre reqs done prior to senior year, or at least spring semester of senior year. There may be a class or two that you're unable to get into until then, but if you wait it could really cause problems due to time scheduling and class conflicts, and also some classes are only available in either fall or spring semesters. If you plan now, you won't have to be stressed out taking a bunch of hard classes in one or 2 semesters after each other.