Hi, as you have less %age in 12th and I can understand what is going in your mind one side you want to improve your 12th marks by appearing in improvement exam and if you're investing one year, you want to prepare for JEE Mains 2021 at the same time. Right!

But the truth is you can do one thing at a time whether you want to prepare for JEE Exam or 12th. So first of all decide what you actually want to get if you invest your 1year or what you will have after 1year. So I personely suggest you not to drop your 1year just go forward in your life with the marks you have scored. Try to prove the thing that marks doesn't show your true potential. OK.

Good Luck.