Hey,
From the starting of class 11, it us the best time to start preparing for IIT. As you'll have enough time to prepare,  also you'll be parallel learning the boards concept and then taking that concept to IIT level.  Also burden would be less as you'll have 2 years to prepare whereas after 12, first of all you'll have a lot of burden of dropping a year and you'll feel pressurised for clearing IIT.
hope this solves your query.
Good luck!