Don't worry because marksheets do not mention the caste, you need the caste certificate only to avail of the concessions of lower cut-off when you have ended-up scoring less that needed.Talk to the school authorities and while the 12th board registration mark your category as of now. As a proof you give your valid certificate. Submit a correction form with caste certificate with affidavit in clg .If not then you can do it in regional office.After 12 you can fill your OBC category you will get advantage.

Hope this helps you.

Thank you.