It is mandatory to subscribe every month but sorry, there is no instruction found for such case in the G.P.F. Rules for aided Schools. There is a provision for the new subscribers to deposit the arrear subscription which I am quoting below.
2. Deduction from salaries
(a) Deduction shall commence from the salary for the month in which the subscriber becomes eligible to subscribe.
Provided, however, when a subscriber becomes eligible from a retrospective date for any reason whatsoever, the subscription shall commence from the date of eligibility and subscription shall be recovered in one lump, or,in instalments in a number of months at the option of the subscriber where salary for the earlier period has already been drawn and disbursed and the recovery of current subscription and the arrear subscription, if any, shall run simultaneously.
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In view of the above I would like to suggest you to file an application to your D.D.O. for taking necessary action so that the error be rectified.