first working test for classroom_spec with outline of upcoming tests
changed association from users_classroom to users_classrooms
changed method all_assignments to alias :all_assignments
removed unused instance variable definition
removed unnecessary self
Separation of Concerns in add_student_to_class

*removed instance variables
*removed Classroom (because this is already a method on Classroom)
*leveraged ActiveRecord relationships to find a user_classroom without reference to the User class
created rspec tests for the Classroom model