Hi, as a former ISC student myself, I highly recommend coaching. The pattern of our question papers and the CBSE/NEET question paper pattern as well as our syllabus is slightly different. Coaching institutes provide students with well-structured notes covering essential NEET topics and exclusive guidance.They hold mock exams regularly, which increases your motivation and competitive spirit. They solve any doubts/problems you have and clear any concepts you might have trouble with. They give advice on how to improve on your weaker areas. Coaching definitely helped me in all of the above areas.

So if you're going to appear for NEET 2019, I suggest you get started with coaching now. Join a reputable coaching institute that has been around for a while and whose students have secured good NEET results. Try and concentrate on both NEET and your ISC exams (because your 12th board marks remain with you for life, so pay attention to that also). Coaching is an investment, but you will most probably do much better with coaching than without. Your hardwork and sincerity is also necessary, or else the coaching is of no use. 

Coaching + self-study = good preparation strategy for NEET. 

Good Luck!