I've taken the weeder classes at decent colleges and I have to say that "not up to it" simply means undisciplined. While my experience isn't all-encompassing,

I've taken the weeder classes at decent colleges and I have to say that "not up to it" simply means undisciplined. While my experience isn't all-encompassing, I personally haven't seen any undergraduate class that couldn't be passed by almost any of the students with a sustained effort. Some students are more naturally inclined for the hard sciences than others. It doesn't mean they can't do it. It just means they have to work harder at it. Most just choose not to.