Since 6 April 2014, a new employer’s National Insurance (NI) allowance of up to £2,000 per tax year is available in respect of employees which can be claimed through your school’s payroll.
How does this work in practice? If, for example, your employer Class 1 NI bill is say £1,500 each month, then in April 2014 you could have claimed a £1,500 reduction in your Class 1 NI bill (i.e. reducing it to £nil) with the £500 remaining allowance reducing your May 2014 Class 1 National Insurance bill to £1,000. The maximum allowance is capped at £2,000.
There are a few exceptions to which employers can claim (see https://www.gov.uk/claim-employment-allowance for details). However the vast majority of schools should be eligible.