I am studying in class 11(isc board) and in one of the toughest schools in the country
i have heard that the grade drop in class 11 is drastic
can someone please give tips to prevent it from happening(which books/sections i should study more)
i plan to apply for ivies so i’m pretty desperate 
Ivies are well-aware of the drop.
As long as you remain top 10% in your class, it’s okay.
However, there are lots of excellent schools for different subjects that aren’t Ivies. Ivies are just eight among the top 25 schools in the country, along with Williams, Pomona, Stanford, CMU, Kenyon, Bowdoin, Middlebury, Duke, Jhu, Amherst, U Michigan, etc.
You should find two schools you’re sure you can get into, admit 40% applicants, and that you can afford, 3-5 with an admission rate of 30%, and then only add the reaches, those were your odds are low no matter what (that would include all of the above.)