If you are really familiar with the IITJEE material when you near the board exams, it should not take you more than a month to cover what is not there in the IIT syllabus, and be familiar with the type of questions asked in boards.
Expert Chemistry faculty.Produced Best Result IIT JEE All india -04 rank
Dear Rashmi Joshi,
In current scenario it is very important to score in boards as well as in JEE, then HOW?? Yes you have to maintain that balance. You follow a plan which suites your school/college curriculum designed by an expert parallel to boards preparation and JEE preparation. When you prepare complete the concepts from NCERT and work on JEE level problems from a standard prescribed books. All the best.
As far as Maths is concerned, if you are preparing for IIT JEE, then you don't need to worry about your 11th/ 12th school syllabus because the later is subset of IIT JEE itself. If you are concerned about Physics/ Chemistry, then almost 80% syllabus is common in both and remaining 20% (like Semiconductors, Biomolecules etc.) you have to prepare on yourself.