Aren't you a little late?
The way preparation has become these days, even 14 months cannot be deemed enough time to prepare for JEE Mains. Even 14 months is an overestimation of sorts as you'll need atleast 3 months for CBSE preparation.
Does that mean it can't be done? of course not.
The first thing I would advice from my experience is not to set Mains as the target. Target JEE Advanced. Now many would argue JEE Advanced is simply out of range for a majority of students. I know. I was one of those students. But keeping JEE Advanced as a target would improve your understanding immensely. There will be days when some questions will blow over your head. Bear with them, endure those moments and always strive to go into a deeper level.
Secondly, take your coaching seriously. Don't flunk out. Take the weekly tests seriously. If you don't go to a coaching center, join one. I don't mean to discourage you, but your chances of cracking Mains without coaching are highly improbable. Your school won't care if you score well in JEE Mains. In fact, they would much rather if you ignore Mains altogether and focus on boards exam. Would you rather get 75% and study at a top NIT or score 95% in boards and study at Amity.
Third, work hard. Whether it means burning the midnight lamp, missing school days or ignoring friends. I started preparing for JEE Mains when after I realized board marks count for nothing. You still have more time than me. Ignore what your school says and start preparing for JEE Mains