Removed confusing reference to Students in Hash initializer. Initializer becomes assignment and not ||= since @db will always be nil in the intialize method. Changed argument name of method from grade(level) to grade(grade) as this gives consistent indexing into the db variable and removed the || [] as the hash has a default value already set.
reverted grade(grade) back to grade(level)
Merge pull request #1169 from RichardWigley/master

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