Some students learn best when they're able to discuss an academic principle in groups with their peers, or by asking the teacher a question and having it answered directly. It's difficult to do this in an online class, as you're taking the course virtually, and your instructor and other students are not in the room with you. This means you'll need to exhibit a tremendous amount of self-discipline when it comes to "showing up" to class on time or completing projects by the deadline. This is definitely a drawback for students who would internalize concepts much better through live conversation.
In a traditional classroom, a student can raise her hand and have the teacher respond to her inquiry. This is not possible in an online classroom. While your instructor can answer questions about your assignments and projects via chats during scheduled class time, and will likely provide an email address for you to use after class hours, it could be a day or so before an instructor has the time to respond to an important question. At a traditional school, you can simply visit the teacher's office before or after class to get the help you need in understanding the subject, but delayed online information could lead to turning in assignments late, or lower grades due to a lack of timely assistance or comprehension.
Limited Internet access can directly affect your grade, as you may not be able to log on to your virtual class as often as you would like. For working students and those with families, if anything goes wrong with your home computer, it may not be feasible to work on class assignments on your office computer -- unless you're able to stay after work hours, which could get in the way of your other obligations, e.g., picking the kids up from school or running errands. If there aren't many computers in your home, and other family members need the devices to complete work or homework assignments on a daily basis, online schooling may not be the best option for you.
Real-world schools often have strong alumni associations and networks, so graduates can contact former classmates and get inside job, internship and fellowship information. When you're attending an online school, the alumni network isn't nearly as strong, according to Ace Online Schools, due to the lack of personal interaction. This makes it more challenging for graduates of online schools to find jobs after completing the necessary courses, or to receive the support, e.g., workshops and seminars about job readiness, that are sometimes offered by alumni associations. Ace Online Schools also notes that more online colleges and universities are acknowledging this complaint from Internet-based students, and are working to establish stronger sources of support for online graduates.