Q.Is it necessary to read the NCERT for the UPSC?
Well, It is Must, no matter from which background you come.
WHY IT IS MUST TO READ 6th-12th but not directly from 11th and 12th.
Because, imagine how a 6th class student is taught on a particular topic. and how a 12th standard student is taught on same topic. Let’s say Our EARTH in Geography.
or say Democracy in Politics.
When we deal with these type of topics, basics and all are clearly mentioned in lower standards of sixth and seventh. Whichever topic you choose, there will be extension of those topics in Senior or higher classes. But basics will not be given there. Introduction part will have very few lines where you can’t get whole information.
If you sit for a Month, exactly for 30–31 days, regulary reading for six hours (Excluding Newspaper), you can cover NCERT of any two subjects completely from 6th-10th). Next 30–31 days for another two subjects. So these initial two months will save your HOURS of time in Future. Initially you may feel that these basics are helpless. But as you keep progressing and go for standard books, there you will understand the importance of them, because they will be become a cake walk for you, if basics are strong.
60 days is a huge time, but it will save your time, double of it.