If you're on the school's PAYE or work through an Umbrella Company, you do not need to pay NI contributions after you reach State retirement age. This age varies depending on your individual circumstances - please see https://www.gov.uk/tax-national-insurance-after-state-pension-age/stopping-paying-national-insurance to see what age this is for you. The effect is that you take home more of your pay as the deductions reduce.