Need help, Can a school management legally and ethically withhold a teacher's Experience Letter if they resign after completing the academic year but missed the specific contract notice deadline?

An Early Years Teacher completed a full 1-year academic year under a probationary contract at a reputable private school. ​According to the contract, to resign during probation, the teacher was required to submit their resignation by March 31st to serve a 2-month notice period until the end of May. Instead, the teacher is resigning now in June without serving that specific 2-month notice. ​however, teacher isn't going yet to administration and she is assuming that because the resignation timeline was missed, it constitutes a breach of contract. Consequently, they may withholding the experience certificate and stopping the June/July summer salary. ​The teacher fully served and taught the entire academic year diligently. Missing the administrative resignation deadline shouldn't wipe out a whole year of hard work and documented experience. ​Is the school's decision to completely deny an Experience Letter? What would be best option for teacher either to request the administration else what would be experience count and can mention in resume?