Yes. It's the way all the students do to reach their target. You can finish your 12th syllabus till december and then the rest of the time can be utilised to revise the whole syllabus. Plan your timetable wisely depending on your target. JEE Main and Advanced are entirely different and follow entirely diffeent strategies. Hence make your plans accordingly 

All the best.........

It is better you start studying from 11th. In 11th clear all your concepts and doubts from NCERT 11th Standard textbooks of Physics, Chemistry and Mathematics. If you start from now itself it would be better for you in future. 

In 12th everyday keep revising each chapter from each subject that you studied in 11th. In 12th you have to do the preparation for Boards side by side. So if you would have already finished 11th portion perfectly then you would be able to concentrate on 12th too. In 12th give equal importance to every subject. This is important because if you do only one or two subjects then other things would pile up.

Manage your time. Don't waste time unnecessarily. There are many books available for JEE Mains. Keep in mind that whichever book you read or solve problems from you are thorough with them. Don't keep changing different books. Select one and complete it. 

If you still have any queries feel free to ask in the comment section down below.