When I teach students of Class 11th, even an average student passed out of class 10th from ICSE board has better understanding of math and science than most of the 10.0 CGPA students of CBSE.
The science and math syllabus of class 9th to 12th is far better and scientific than that of CBSE. Some of the concepts given in class 9th and 10th books of ICSE board are so precise and clear that are not given even in CBSE 11th 12th books . (If any body wants to get into more specific, I would ask them to compare definition of work and Centrifugal force in the 10th ICSE book, with the what is given in Class 11th NCERT.
Hope you understand,