What is taught in a formal school cannot be replaced by self study or some coaching institute. Formal schooling gives you exposure and peer learning. Also, JEE main is mainly NCERT syllabus which is exactly what is taught in class 11 and class 12. You can continue your schooling and maintain the minimum required attendance and use the days you take leave for preparation. Most schools also finish their syllabus by December and you would have 3-4 months ro prepare for your boards and JEE april session.

Joining a dummy school may sound convenient for studies as you will have the whole day to you, but it can bring in lethargy and laziness you think you have a lot of time to prepare and end up wasting it. Only if you are very sure of yourself and the hardwork you are likely to put in should you think of opting for a dummy school. But remember, not everyone who opted for a dummy school qualified in JEE Main too.