school.rb

class School
  attr_accessor :classes

  def initialize
    @classes = 7.times.map do |i|
      {
        grade: i + 1,
        students: []
      }
    end
  end

  def students(grade)
    @classes.find { |c| c[:grade] == grade }[:students].sort
  end

  def add(name, grade)
    klass = @classes.find { |c| c[:grade] == grade } 
    klass[:students].push(name)
    klass[:students] = klass[:students].sort
  end

  def students_by_grade
    @classes.select { |c| c[:students].length > 0 }
  end
end


school = School.new

%w(Jack John).each { |student| school.add(student, 6) }

school.add("Dillon", 1)