How are the expectations for students different between high school teachers and college professors?

There are profound differences between the expectations of high school teachers and college professors. Many people have noted how strict high school teachers tend to be in relation to their college counterparts: they assign much more homework; are typically less lenient about deadlines, missing work, and mistakes; and demand a much higher work ethic. These differences stem from the fact that high school is mandatory and is intended to prepare the student for higher-stress environments, whereas college is a choice—it is not required, and its intent is to educate the student in a certain field.
The college professor will typically treat the college student with a much greater level of leniency and will not assign as much homework or mandatory assignments, because the student is choosing to be there. If the student truly cares about the class or assignment, they will pursue it and make a good effort to complete it and get good marks. A student in college is expected to be self-motivated; by choosing to attend college, they have taken the responsibility upon themselves. In high school, it is the teacher's responsibility to get a student through successfully until graduation and prepare them for a potentially high-stress career or strenuous academic activity. Since it is mandatory at that age, the responsibility for success is placed upon the teacher, which is why they tend to seem more demanding and less lenient.