You don�t have to drop a year for JEE. You can start preparing for it as soon as you complete your 10th and get ready for your 11th. The best way to prepare for JEE is to get enough time for preparation, digest all the topics available and be able to prepare for the exam well before the scheduled time. If you do these things as you go through the 11th and 12th, you don�t have to drop a year for the exam.