Are virtual schools effective?

During my research I have found that virtual schools started in the 1990's. The 1990's is when computers started to get better, which would make it possible for virtual schools. Before computers virtual schools used postal services, radio, and television. 

  In the United States, there are 45 states that offer virtual schooling. In 1997, Florida was the first state to have a statewide Internet-based high school. 

There are two types of virtual schooling. One type is synchronous. It is we're all students are present at the same time, which resembles a traditional classroom. The second type is asynchronous. It is were students access there learning on there own schedule. 

Since, I have found the history of virtual schooling, I think it will help me better understand virtual schools.