First of all you should think how serious you are preparing for JEE . Are you not joining coaching because you think you are capable enough for preparing on your own or do you not want to join coaching because of reason like lack of time , lack of confidence or the hardwork you have to face ?
Well you will be in class 12th now , how did you study in your 11th grade ? Did you solved any IIT/AIEEE books ? IF you are just starting , then you need to give time to both class 12 and 11th topic as well as CBSE subjects . Now this may sound hard managing it , but its not when you actually start preparing . Solving Tricky and good question definitely increase your self confidence and will help you in your school subject .
Let me tell you my story how i joined coaching ( FIITJEE in 2011) , when i entered class 11th i was very irresponsible , didn't know anything about IIT/AIEEE that time . I came to know that many of my friends have joined various institute for JEE preparation . I was little jealous when they use to study and discuss chapter 6 of maths book ( CBSE) by themselves when i was doing chapter 1 . So i joined coaching just because all others have joined , without any motivation . That was the worst decision i made . Atleast that's what i thought in first few weeks because it was hard to manage school and coaching . I was an average person in my batch ( maybe among bottom 10) , but little by little i moved forward , realizing how big the competition i have to face , how much i have to improve , how much i have to give more time . By the end of phase 2 ( after 8 months ) , i was among the top 3 in my batch . Definitely joining coaching helped me a lot in 1.5 year . Their study material definitely increase your confidence . So i will highly recommend joining a coaching institute , but you should promise yourself first to be hardworking and very serious when it comes to Engineering Competition .