I agree with this as the father of two kids in high school. I routinely force them to fully understand some things they learn, and I am more liberal with other

I agree with this as the father of two kids in high school. I routinely force them to fully understand some things they learn, and I am more liberal with others. Mathematics is one of these subjects where there is almost nothing you can afford not to understand. It will absolutely bite you someday. When learning math there are things that are really new (such as differentials, or operations on fractions) that are conceptually different from other things you just learn (Pythagoras theorem for instance) and I spend a lot of time with them to help them understand the "why" of these operations. Otherwise i know they will have a very, very hard time to follow up. Physics is lighter - you can have trouble understanding thermodynamics and it will not mean much when doing mechanics. There are parts of physics I never really understood despite having a PhD in physics. This is in sharp contrast with, say, history where missing a bit does not impact the next things one learns (at least at high school level - and this is from someone who loves history).