Hello aspirant,
If the school is affiliated to CBSE then the mode of education maybe  English or Hindi and this depends upon the teacher or the institute.
Your institute teaches you in Hindi maybe for better understanding of the concepts but the paper has to be written in English only. The cbse board paper has to be written in the language that your institute permits to be.
It is either English or Hindi and that depends on the institution and their common language.
Hope it helps