The main challenges that students face in such classes fall into two categories.
The first is in understanding the material. The material in such classes is typically conveyed only through written material. This means that all the information that a student gets has to be obtained by reading. Particularly in high school, many students taking online classes are not strong in this area. They may do worse than if they were able to hear a teacher actually speaking about the material.
The second is in keeping up with the work and being motivated. Online students are typically on their own when it comes to doing work. They do not have to attend class every day, thus forcing them to at least listen to discussions of the subject matter. They may be allowed to work at their own pace. This makes it very easy for students who lack motivation and/or time management skills to fall behind and risk failure in online classes.
Some students do extremely well with online classes. This is an alternate type ofschooling for those that do not fit into the traditional classroom, or need to be able to do a course of studtheir his/her own timing.
One of the major issues for online learning is that the student doesn't get the benefit of class discussion. In class discussions, a student gets the benefit of hearing other points of view, stating his/her point of view, and the opportunity to refine his/her point of view accordingly.
Time management is also a problem with online classes. A student needs to be organized and motivated. It is so easy to start working on something else or to go watch TV than to work on assignments. Next thing the student realizes is that he/she is getting behind.
There are pros and cons to online classes. Eacin person needs to weigh the pros and cons for themselves.