Definately you must have to start your preparation from 11th class ,but to take admission in IIT college you must have to fulfill minimum criteria of percentage in class 12 so you give priority to board and also continue your preparation
Yes just make your concepts stronger and for IIT JEE if you have sufficient time then you must go for Olympiads and other exams preparation based on your foundation course.Thisight help you to achieve better knowledge.
