If you want to start graduate school right after you complete your undergrad degree, definitely begin applying this coming summer. I will be graduating in May, and starting PT school in June. I started my PTCAS application in July even though I still had (and still have) to complete A and P 1 and 2. The programs that I have been accepted to all accept me on the condition that I complete these courses, but they accepted me nonetheless. As long as youve complete most of your pre-req classes when you begin your application and go on interviews, it's okay if you have one or two left to complete.