Since you are now in class xith, it is high time you prepare for iit as board exams will be one year later so you can completely dedicate yourself studying for iit-jee, just bunk school as getting high marks in your school will not be of much use and take out of time to study for atleast 10 to 16 hours a day. However do not miss your school practicals as the past few years has seen a rise int he no. of question asked in experimental physics and experimental chemistry in JEE & AIEEE and this trend is likely to continue. When you get to XIIth do try attending school frequently if not daily as the board papers are often predictable and the school teachers have a good idea about their pattern, also try to give a reading of the chapter from NCERT before it is taught in class. Xiith chapters are more scoring than xiith and unlike Xith the board chapters of xiith have more relevance than that in xith. Also remember that you can always appear for jee second time but boards only once. What policy JEE will adopt might be revealed this year IF they give weightage to XIIth marks you should go for the boards as you can always give jee next year( i got 94.6% in boards and i knew i was safe and i gave jee again and i m scorin real good) and if not forget boards and study for jee.