Hello Anshu,

IISER has clearly stated its eligibility criteria for admissions:

Candidates who have passed 10+2 or equivalent level exam with science stream in 2018 or 2019 are eligible to apply for admissions. Candidates will be admitted to IISER only through following three channels.

1. Kishore Vaigyanik Protsahan Yojana (KVPY) channel
2. Joint Entrance Examination (JEE-Advanced) of the Indian Institutes of Technology
3. State and Central Boards Channel (SCB)

Hope this helps.