Nowadays open school are so popular with their benefits that government approved open board students to be eligible for national and state level exam so you can apply for b.tech from bits university.there eligibility criteria is you should passed the 12th class examination of 10+2 system from a recognized Central or State board or its equivalent with Physics, Chemistry, and Mathematics and adequate proficiency in English .

There is a high chance you can apply for B.Tech at BITS Universities (Birla Institute of Technology and Science) with a 78% PCMB score from the Board of Open Schooling and Skill Education (BOSSE), Sikkim.
BOSSE is recognized by the Sikkim government and the Association of Indian Universities .This recognition validates its certificates for higher education purposes in India.

Therefore, based on these points, you should be eligible to apply for BITSAT and potentially secure a B.Tech seat at BITS Universities if you score well in the entrance exam.

https://www.careers360.com/university/birla-institute-of-technology-and-science-pilani

I hope it helps!

I hope this information helps!