To do or not to do

Primary education, a compulsory education in Singapore. Some may progress well and some may not. Math topics move on one after the other in primary education. What if some students are lacking far behind from what the school is teaching in math lesson? And clearly, they have little understanding of the math concepts taught but are forced to move on. We all know that students will still be promoted even though they do badly for exams. How should I allocate time during tuition to help strengthen the student's foundation of mathematics yet still able to keep up with the school?