It is a genuine case. So, you can get a duplicate copy of certificates. For this your personal approach is necessary instead of writing correspondences.
You can approach the school Head Master for SSC certificate and Principal for higher secondary certificate with your proof of Identity (anything like aadhaar or voter id) and it is better if you have any proof to prove that you are the ex-student of that school. It is better to see whether any of your teacher still working there and if you approach them the process would be very easy as he identify you to the HM/Principal as ex-student. Sometimes your teacher may be HM/Principal at the time of your visit.
When you collect the duplicate certificate verify whether your Name,father's name,date of birth mentioned correctly. Name, father's name, Date of birth are the basic details which required till our last breath for anything to be proved and they should be similar and a correct one in all documents. The school certificate for seniors and birth certificates for youngsters become a valid one. As the date of birth is deciding matters in one's life till his end,much care should be taken in the beginning itself. School certificate, college certificates, aadhaar, Pan card, voter Id, driving license should tally each other.
You immediately approach in person to the authorities without delay. No correspondences will yield good result rather than personal approach. Your delay will make the matter more delay.