I can only talk about math in our schools of course, so dont forget that. But What I realized is, that math teachers here do a huge mistake. They teach you a wrong way of thinking. Understanding the principles is not even that hard, we are still talking here about "normal" math, you have formulas and the procedure is very often the same. Like curve sketching. But for me, the problem is, that you require a different approach when you get a test or exam compared to how the teacher is teaching you math. If that makes any sense for you. I have a hard time to explain it right now. To give you one example, they explain you something, giving you a formula and how to use it, but in the exam you are required to decipher the math problem they give you, its like they speak in the exams a completely different language, hence why even people in our class that have a very good understanding of math, have a problem in exams, and it seems this is not only a problem in our class but a problem in general.
I think one source for that is, in our class for example, that we only spend 2 hours for math every week, while it should be actually 4 or even 6. 2 hours (aka 45 min per hour) is really like nothing ... thats barely enough to explain the stuff. But not enough time to actually practise. And you cant do all the practise at home.