I'm sorry but that "you'd better be fucking serious" statment sounds more like propaganda than truth.
You DO have like 1 class every semester you have to be somewhat serious about, but I can't tell you how many classes I had where I missed more than half attendance (cause so many professors don't even take attendance) never had a single day of homework, and never really had to do anything but study for a couple days before the mid term and final.
I go to a competitive private uni, known for being rather hard to get into, but I'm not really smart at all. In fact, I think in your average american university a chimpanzee could get their bachelors. There are definitely certain professors who will make you work for your grade, but for the most part I wasn't that serious at all (at least till doing my thesis you have to be somewhat serious about that) and wound up with a 3.2 gpa. And this wasn't just this one uni it was also the 2 year community college I went to before that, and most colleges that my friends attended.
Not trying to give the wrong idea but college is more what you can and can't get away with than being serious as all hell about it. Once you understand the basic logistics of specific professors after your first year, its rather simple to sleep through half a semester and still ace most classes in your sophmore year. Anytime there was a professor who gave homework everyday, or gave impossible exams to pass, they were usually the ones with 20 empty seats come add/drop week. In fact, out of the 128 credits I acquired, I can only name 3 classes I actually had to be serious about. Research methods, stats, and senior thesis. But maybe its different for other majors who knows. I do know when I majored in chemistry organic chem 2 was a nightmare, but I still believe at least 75% of most classes are a joke.
Not to argue just stating me experience.