Hello, There is 4 year BSC program in IISC. You can choose your specialization only during the fourth year of your degree. For admission you gave to clear on of the following exams after your 12th board -KVPY; IIT-JEE (Main) or (Advance);NEET. Since the syllabus all these exams are similar to your 11th and 12 the board, it is necessary that you have a clear understanding of basics and solve application oriented questions alongside. Focus on both 11 and 12th parallelly for a topic so that you have both foundational and in depth knowledge. You can visit this link to know about the admission process in IISC https://www.iisc.ac.in/admissions/faq/

Best of luck!