Here's what worked for me, and this echos more than one other commentator:
Write down contemporaneous notes from the lectures. For me, like "what", this commits it to memory, I hardly ever referred back to them.
For math based subjects, do the problem sets as they are due. In these subjects, things tend to build upon themselves, so this is not something you can do at the last moment. Even if there are no formally due problem sets, do enough work so that you really understand the material; if you don't, you're just fooling yourself.
I never "crammed" at the last minute (although one disaster marred term had me learning the last of the material in one subject in my normal fashion and pace the night before the exam, which I then aced), but be as sure as you can to get a good night's sleep before the day of the exam.
Anyway, as you note, in high school it may not matter so much, but you do have to get your foundations one way or another if you're going to go on to college in a math based major. So get serious about establishing your foundation before then. There were a few gaps in mine (I went to a low quality high school where my last math teacher simply refused to teach his class) and they caused quite a bit of suffering when I got to college.