Dwar Baite, for entrance to any IIT among the 23 IITs JEE is a mandate entrance examination. You can try for Management seats but the fees will be too high. If you have 80% and above in your 12th, it shouldn't be a big task to score a decent mark range in JEE Mains. The competition is too high but it is still affordable than the college fees under the management quota. So just to answer your question to get into any IIT, JEE mains is mandate and your 12th percentage isnt enough alone.