You can now select 'Manual  Class Creation' from the Tools menu when creating classes.  This will place all students in the grade level selected into 'Unplaced' (image below), and so you can drag and drop students into their classes.
 
We have created this feature for schools that would rather "start from scratch", but still have access to the surveys, colour-coding, alerts and student history.
 
 
After selecting Manual Creation and clicking Re-Sort: