Classroom does not display properly for students in new admin

[americac]

[gr4yt3x]

I think this is related to #984.
Some parts of the codebase are still using the old classroom.grade field instead of the new classroom.grades association.

The error in the student view seems related to format_attribute, which is called by the admin_show_attributes helper. When grades is passed in, it returns an ActiveRecord::Relation instead of a formatted string.

Previously, the Classroom model exposed a grades_display method that returned the properly formatted value. That method may be useful to restore or reuse here.