Dear Student ,

Generally , A Specific board (state/national)  allows you once in each class if you pass the previous class or unless you fail. If after qualifying 12th exam .. you want to do 11 and 12 again then you have to change the board and in new board you have to check the eligibility for admission .

For IIT admission. You must be from Science background in 11 and 12th with a standard passing marks .

Then you have to seat for JEE MAIN  and after qualifying .. you have to appear for JEE ADVANCED and based on the cut off .. you can get admission in IITs for individual B.Tech Courses .

Hope this helps !

For more details, you may refer here :