Hello Manish,

With unprepared 11th it is of course possible to clear your 12th regardless of what board you are in. The syllabus of 12th is, somewhat if not completely, independent of 11th and with weak background you will take a little extra efforts to clear up 12th with respectable marks. It is however a very hard nut to crack if you try JEE with this level of incomplete preparations. Even if you clear the cutoff , you might not be able to land up a good college or a good discipline or even a college at all. So it is advisable to focus on at least ⅓-½ syllabus of 11th in these summer gap .and let me mind you that you will need 11th Mathematics to be clear for 12th one. 

I wish you all the best .