See there are no information for repeating class 12 after passing it available on the BIHAR HIGHER SECONDARY BOARD website. So I would recommend that for the procedure to continue you have to contact in your school and you have to register yourself as a new candidate from the beginning i.e., from 11th standard. So you have enroll yourself for the 11th and start over again, which I personally feel is quite a lot of time investment. It would be better if you can get a one year schooling system with Biology. You can go for the open school such as NIOS etc. Although there are cases on the Supreme Court regarding the credibility of the NIOS students to sit in the examination such as NEET but as there is no verdict available till now, you can obviously go for NIOS.

To know more about the NIOS visit the link below:

https://school.careers360.com/exams/nios-class-12

I hope this answer helps. All the very best for your future endeavors!