Why is attending a class in person better than attending an online
class?  I need four reasons from documented sources for a speech.

It's much easier to ask questions of the instructor when the
instructor is physically present. Online classes may have less
give-and-take between the instructor and the students.

A good instructor can tell if the audience is following the material,
and adjust on the fly. Ideally every time a lecture is given it is
tailored for the current audience. This is much harder to do online

"An experimental design was carried out during the Fall, 1996 in which
33 students in a Social Statistics course at California State
University, Northridge were randomly divided into two groups, one
taught in a traditional classroom and the other taught virtually on
the World Wide Web. Text, lectures and exams were standardized between
the conditions. Contrary to the proposed hypotheses, quantitative
results demonstrated the virtual class scored an average of 20% higher
than the traditional class on both examinations. Further, post-test
results indicate the virtual class had significantly higher perceived
peer contact, and time spent on class work, but a perception of more
flexibility, understanding of the material and greater affect toward
math, at semester end, than did the traditional class."
http://www.csun.edu/sociology/virexp.htm

Here is a study from California State University at Northridge about online classes.