Hey Amit , You don't need to appear in Class 12 Board exam for giving second attempt in JEE Main and Advanced You can take One Year drop and prepare for JEE

But keep in mind one thing that in Class 12 Board exam you should have atleast 75%  ,because it's a criteria for sitting in JEE if you don't meet this criteria You can take improvement examination in board to get this criteria to be fulfilled . BEST OF LUCK