Should I drop a year for the preparation of JEE as I want to study in IITs only?

If you are willing to study in IIT colleges only and if you think that you will be able to crack JEE Advanced next year, then yes, you should take a drop.

Drop is not necessary step for IITs. Many IITians haven't taken any step. If your focus on your goal is clear then you can also clear IIT JEE without drop. Your concepts must be clear whether you take a drop or not.