Hi
Usually students don't focus on their class 11th as it's all a new stream plus the course is huge which is a major jump from class 10th course so do not worry.
Since it will be a board year number so you got tons more focused on the course material of class 12th but with that you need to know some few chapters of class11th.
Though if asked for wise guidance and time permits you must go through your class 11th subjects once.
This is only applicable if you just want to get good grades in boards.but if you are preparing for some entrance exams too then you must have a thorough grip of all your chapters from class 11th and class 12th.
I am enlisting the chapters of class 11th you need to go through once for a good understanding of class 12th in respective subjects-
BIOLOGY-
Chapter 8- cell the unit of life
Chapter 10-  cell cycle and cell division

CHEMISTRY-
Chapter 1- some basic concepts of chemistry
Chapter 3- classification of elements and periodicity in properties
Chapter 12 -organic chemistry
Chapter 13- hydrocarbons
Chapter 7 - equilibrium

PHYSICS-
Chapter 14 oscillations
Chapter 15 waves

All the best.
Hope this proved to be of help.
Feel free to ask any other query.