I perform well in Chemistry while giving mock test for JEE and poor in Physics, while at scholl exams I perform good at Physics and average in Chemistry. How should I balance both at both levels.

@ mudit 
that is the main problem , you are cramming physics , not understanding it . 
see, in school exams they test your representation skill , so, if anyone can cram a concept and then write it on the exam paper , good marks can be possible in physics for school exam , and u are not performing well in chemi beacuse chemi u cannot mug up , and reactions are there in class 12^th . 
so, for entrance u need to study diff books , like h c verma for phy , and o.p tandon for chemi , and board can be done by ncert only . 
and 1 more thing – dnt cram physics , with cramming u cannot solve a single ques even in jee advanced . 
so, understand by concept . 
ALL THE BEST ..