Hi, 

My brother gave NTSE exam I think I could help you regarding preparations as I have closesly observed his schedule.

Saturday and Sundays are jackpot days for you utilise and manage it efficiently by preparing a strict timetable prefarebly most studying chunk to be finished in early morning hours. Clear your basics from ncert ( if cbse syllabus) for math or bio.,  chemistry and physics. 

Practice those textbook sums as many times as possible. Give importance to concept understanding. 

For SAT give 1 hour each thrice on weekdays and MAT 1 hour each twice in weekdays and 2 hours on holidays but for either SAT or MAT. 

For boards make time table taking two subjects each day apart from revising topics taught in school on that day. Try and complete assignments of school on time so it doesn't cause burden on single day. But study 2.5 hours each on week days and stretch it to 7 hrs on weekends/ holidays.