For context, 2% is also the acceptance rate of all YC applications. I'm guessing that people who apply to Startup School tend to be very early in the founding

For context, 2% is also the acceptance rate of all YC applications. I'm guessing that people who apply to Startup School tend to be very early in the founding process, which gives them an a priori lower chance of getting accepted (even though YC is open to super early stage), and then SS raises it. Also, the kind of people applying to Startup School are probably not the kind of experienced founders whose application looks more promising at the very early stage.