TSA uses a holistic review process, meaning they consider more than just grades. Auditions or portfolios are required for many programs, and academic performance is also a significant factor. Therefore, a strong application showing both artistic talent and academic ability is key to increasing your chances of acceptance.
In short: It's not *easy*, but it's not impossibly difficult either. Success hinges on the strength of your application and the specific program you're applying for. Checking TSA's website for specific admission requirements for your desired program will give you the best understanding of the level of competition.