Some professors use television to supplement their lectures. There are some courses in which the lectures are videotaped and made available to students for review or if they missed a class. A full-time load is 12 hours. It's certainly possible, but most students nationwide don't graduate in four years. At Ohio State, many students choose to participate in cooperative education programs, internships, and study-abroad programs, which will delay graduation beyond four years.
Many students also choose to take off a quarter or two to work or to meet family obligations.
At Ohio State, there's such a wide array of courses to choose from that some students stay longer than four years just to take advantage of all those opportunities. Both professors and graduate students teach at Ohio State. Graduate teaching assistants are used primarily in introductory courses. They're required to take courses on how to teach, and since they're studying for a master's or doctorate in that particular field, they're very familiar and comfortable with the material. In many cases, they're from the top of their classes and are certainly well-qualified to teach courses.