It is interesting that they are looking for "high school students" and casting 18-30 year olds. Apparently, this is an acknowledgment that the principal cast of kids are all about 20 years old now, and would look "old" if you cast real high school students to be extras. So, you cast "youthful" looking young adults to surround the main cast...
By the way, they typically film episodes pretty much in order, so this casting call strongly suggests that there will be some sort of "school" scene in episode 1.