It would be better if you start your JEE preparation after your Class 10 boards. You can take IIT coaching alongside your school if you want as the courses taught in both the places would almost be going and in hand. One small advice – learn the basics of differentiation and integration before you start with your Physics as you would need it.
In my honest opinion, the best time to start preparation for JEE Advanced, as it is now known, is after the 10th standard board exams. There are many coaching institutes which begin preparation as early as 5th and 6th standard, but this is highly unnecessary, and in my opinion piles pressure on the child when he should be outside playing and enjoying childhood. Two years is more than sufficient time to prepare. All you need is focus, determination, and hard work. Good luck!