As long as your Peace Corps application process is under way by the time you arrive on campus at MIIS to attend classes, you are officially considered a PCMI candidate. You are not required to have already been accepted by the Peace Corps. If, however, you arrive on campus to begin classes without having submitted your Peace Corps application, you will not be considered a PCMI candidate.
If for any reason you are not subsequently accepted by the Peace Corps, you are still considered a graduate student at MIIS, but will no longer be considered a PCMI student. This would mean that your third semester at MIIS would not be subsidized by the PCMI program. However, you can still complete your MA TESOL degree at MIIS with no time lost and no additional requirements.