It depended on a few things for me personally.
I made it through several years of a liberal arts degree with a 3.5+ GPA by showing up to lectures and doing the bare minimum required on assignments. I did very little homework. Mostly, I showed up and paid attention to the lectures. I felt they were the most singular part of the college experience that I was not likely to find elsewhere, they were generally interesting, and you could, if you were lazy like I was, skip most of the required reading if you paid attention to what the professor said.
High school was different, the teachers are focused on orienting you toward the material in a more remedial fashion and classroom time was, at least for me, an exercise in boredom and frustration. Students also don't necessarily know how to learn at this age, and some need a lot of step by step guidance and repetition. Some students require personal attention from the teacher. Most of the coursework is covered outside of class, in the homework.
For highly technical topics there is no substitute for homework - you need to drill to make the material your own. For more subjective subjects, however, there is little reason to do a lot of work outside class unless the teacher is particularly bad or overloaded with needy students, or you have an exceptional passion for the material.