You should first focus on your 12th board and read NCERT and related books as much as possible for a given chapter in a given subject. After you think you have gained confidence over that chapter, go for JEE level question practice sheets and try to solve it. As your school runs JEE classes also, so you can ask any doubt on JEE level questions from your school teachers. Also, the most important thing is study daily on a regular basis and keep a time table.

You should concentrate basically on class 12th NCERT syllabus. JEE mains exams  contains mostly NCERT textbook questions. Go through each and every line of the topic thoroughly.