With current employer you should build favorable written record under proepr acknowledgment for use at appropriate time in approrpiate forum.
With next employer too you should assert in writing under proepr acknowedgment that you can supply copy of notice of resignation with prrof of its disptach and acknowledgment and delivery (hope you have it), salary slips of past months,form 16 etc……………………..and can supply acceptance of resignation/service certificate/relieving letter only if and as and when it is supplied to you.
The notice period of 90 days shall be inserted in the appointment letter,as per rules/guidelines/enactments applicable to establishment e.g. CBSE affilliation rules (if it is affilliated with CBSE), State Education Act, Shops & Commercial Estb Act if it is a Estb for commercial gains (in that case it might be necessary to register also as per the Act). You can approach them and various teachers associations/forums and escalate to authorities above principle.
The relieving letter signifies that nothing is pending at the end of employee as rightly under stood by you.
The 90 dasy notice period is sufficient for the Principle to put his house in order and arraneg to complete all of his internal exit formalities and supply FnF statement computing earned wages/leave encashment/bonus/Gratuity etc and payment of FnF wages,PF a/c statements,Form16,service certificate,Releiving letter etc……………..on last day in office.
You should cover in notice/subsequent communications that NO Tasks/nothing is pending at your end and arrangements be made to complete all exit formalities and all above metnioned documents be supplied on last day in offcie i.e. dated……………….
You may also add that you require ………………………on last day in offcie i.e dated………………………so as to mproduce the same ot next employer on dated…………………..to join the duty…………………….and until ……………….is supplied to you and hence next emploer…………..on dated……………..you will not be allowed to join duty and offer/employment shall be terminated……………………
You may add that on dated…………….Mr/Ms…………..has stated that relieving letter shall be supplied 30 days after LWD that is without any reason/logic and determinetal to your employablity………hence it should be supplied to you on dated………….