Prepare simultaneously for JEE Main & Advanced strongly and try completing the Book "Test of Mathematics for 10+2 Level" by ISI Kolkata besides attempting Previous Olympiad Questions and so on. Try appearing for RMO and KVPY, preparing well for each of them, will give you confidence. I believe youll make it into ISI B.Math Course, provided your concepts are strong enough. You have a huge advantage nowadays with the numerous Free Material Available over the Internet as well as MOOCs and Courses.