I was also a very poor math student in HS that went on to get a physics degree. I can say that it was the method of instruction as well as my own lack of motivation. Classes were boring and easy to pass even if you did not know the material. In my state you needed a 2.0 GPA to get into a state school. This is generally doable for even the worst students.

The lack of real consequences for not studying and doing homework played a huge role in my childhood academic experiences. I am often baffled when I hear reports of students who are over stressed by the requirements of high school academics. The standards seem now to be even lower than when I was in school yet students constantly complain that they can't keep up. It's mind boggling.