If You are going totally determined for JEE then you need not worry about about you board exams. While preparing of the JEE exams keep fundamentals of all subjects clear , solve the past years papers to get an idea about the toughness of the JEE Mains exam and you gat an idea about time management too.

Pay full concentration while studying NCERT so that this will help in clearing fundamentals. Try  solving more numericals so to have a practice.

Also study the non jee subjects like English or Social Science subjects on weekends on a daily basis so that it won't be a burden at the time of examination.

The board exams won't  be of much trouble for you if you prepare with full hardwork.

Thank you & All the best!!!!!