In class 11 and 12, we study, learn and practice questions for maths topic wise. In JEE, different concepts are used in one question to solve. Also, JEE is an advanced version of Class 11 and 12 Maths.  

In JEE, there are a lot of chapters included in Mathematics syllabus and unlike Chemistry or Physics, there is nothing called less important or very important in mathematics as most of the chapters are interlinked. Therefore you can not skip any particular section to focus more on any other section.