Hash assignment using 'students' confusingly, unnecessary ||= in initializer. Grade has || when a default_value has been set in initi.

Removed confusing reference to Students in Hash initializer - db used throughout class and students overloaded as used to describe the hashes values. 
Initializer becomes assignment and not ||= since @db will always be nil in the initialize method. 
Removed grade's || [] as the hash has a default value already set i n initialize method.
Changed argument name of method from grade(level) to grade(grade) as this gives consistent indexing into the db variable (opinion)