Yes, a student can be in more than one class at a time. Each teacher will be able to see the student in their class. A student's progress and activity will be reflected in the data for all classes they are enrolled in.
School averages are based on unique students at the school - so a student being in multiple classes will not affect them.
Each class can have a separate list of assignments. When students log in, the student can choose which class's assignments to play.
Students can only have one grade-level Journey at a time. Any teacher with access to the student can change their Journey.