Kindly consult your lawyer and issue letter/notice/legal notice to concerned reporting authority/DGM/GM(HR)/Chairman of the bank and all opposite parties including IBPS .If your lawyer advices it may be mentioned that the answer sheets are case documents therefore can not be destroyed and the letter/notice/legal notice shall be submitted to court. You can also include that the bank has expressed passing the exam as a condition in employment contract and thus bank is the custodian of the answer sheets and hence bank has to keep the answer sheets in safe custody and if destroyed it shall be treated as willful destruction of crucial case document/evidence.
Yur lawyer may be in a position to advice better draft of the matter and effective approach to restrain the bank and IBPS from destroying the answer sheets. If possible the restrain may be applied for all candidates.
Valuable advice of learned experts/members is sought.