Follow NCERT textbook of class 11 and 12 

Go through each and every line throughly, solve example problems and attempt to solve back excercise before taking help of your teacher.

Make a note of all the important concept so that u can have quick revision before the Exam, and Make a habbit of writing notes.

Every time u study the same chapter ,make a notes again(so that till the date of examination u have very very short notes , and it make it easy to revise), and write the things which is important and the concept which u think u can forget it later.

If you understand the concepts and follow the above instruction , nobody can stop from scoring in the range of 90-110 marks.

If u have spend quality of time on chemistry and u enjoy the subject then u can expect 100-110 marks for sure