Hi Kunal
As far as my research, the medium of instruction is bilingual. It's not completely English or Marathi. But I would suggest that you should consider some senior regarding this issue he/she might be a better person to guide you.
All the best

The medium of Instructions for MSBSHSE Higher Secondary Education –

The schools affiliated to Maharashtra State Board of Secondary and Senior Secondary Education can adopt any of the following languages as medium of instructions for imparting education to students –

1. English
2. Marathi
3. Hindi
4. Gujarati
5. Urdu
6. Sindhi
7. Kannada