Hey Alok!

I understand your problem. Probably you are preparing for JEE/NEET along with your schools.

The best strategy for this is to take both the things parallel. Like divide the day into two parts and allot each part to either of the two domains. I assume the coaching institute also teaches relevant things, i.e. although different, at last, both the things will be the same. School teaches A, B, C, D.... coaching institute teaches E, F, G, H... but at last, the whole syllabus is A, B, C, D...upto Z. In case you know Venn diagrams in the set, I would explain you that suppose school teaches set A, coaching set B, but your at the last you'll come to know that A is just a subset of B, and most of the part is overlapping. 

I understand, both have their own exams and studying two different things when others focus on one thing, you will have to do 2 days work in 1 day.

As you said, you joined a weekend batch, I suggest you devote time to both domains on respective week days.

Since Saturday and Sunday, you have coaching, just 2 hours on weekends for your school work, rest time for coaching and their homework/revision. 

On other days, 3 hours per day to coaching work(revision and assignments) rest time to school work(homework, test preparation).

You may divide the work and hours according to your priority. Remember the revision of taught lessons is necessary, if that is done properly, and you are able to cope up with two different domains at the same time, you will be ahead of your competitors. This difference in syllabus will only be temporary, like only for 3-4 months. After that, what you read in school will be taught in coaching and vice versa, this will give you an edge over others. So take this as an opportunity to excel over others.

In case you feel coaching institutes are teaching irrelevant things, change the coaching. Remember school is the first priority, not the coaching. As failing in coaching goals, you may appear again, but failing in school can lead to stains in your career.

Hope this helps.

Regards 

If the two syllabi are entirely different then you should opt for the school syllabus because the school is an authorised body and will provide you with certificate. If they are somewhat different then try to make a balance between the two or go for that path which seems convenient to you.