Dear Lavanya, you can pursue a single course in Mathematics from NIOS separately. They have On Demand Examination System. You can read about that here at : https://www.nios.ac.in/on-demand-examination.aspx.

As you are already studying your 12th, I would suggest you to get in touch with any nearby NIOS study centre to check if you can take up Mathematics as a single subject simultaneously along with your regular school. Or you can also reach out to their helpdesk.