Only in a small group a person can develop properly. At school, students learn how to make friends, be patient, get rid of disappointment, and especially to compete. Competition between colleagues can be very stimulating and students will only benefit from it. Online learning cannot offer human interaction.
Another disadvantage refers to the fact that online courses cannot cope with thousands of students that try to join discussions. Also, online learning can be difficult, if it is meant for disciplines that involve practice.