Hi satellite,
You may be okay.
AT
http://in.vfsglobal.co.uk/OCI.html
On the DOCUMENTS REQUIRED tab:
UNDER "Documents required for Applicants born In India" it states:
Applicants who have held an Indian passport, and obtained foreign
nationality before May 2010, but are unable to produce the same for
Surrender of Indian passport for any reason (lost/misplaced/submitted
to UK Home Office long time back) are required to furnish a notarised
affidavit in original from UK stating the facts about their Indian
passports along with other necessary documents as mentioned below
AND UNDER "Other necessary documents of Indian origin" the 3rd option is:
School Leaving Certificate (in original). Post-dated School
Certificate/School Leaving Certificate from India should be
authenticated by Director of Education/District Magistrate/District
Collector of the area with name, stamp, seal and contact telephone
numbers with area code for verification
So if your father's school education certificate is a school leaving certificate
you should be okay. Note: If the certificate is not in English or Hindi they
will probably ask for a certified translation.
Good luck.