gistfile1.txt

class Klass < ActiveRecord::Base
  has_one :teacher
  has_many :students
end

class Teacher < ActiveRecord::Base
  belongs_to :klass
  has_many :students, through: :klass
end

class Student < ActiveRecord::Base
  belongs_to :klass
  has_one :teacher, through: :klass
end

k1 = Klass.create(name: "Room 1")
k2 = Klass.create(name: "Room 1")

s1 = Student.create(name: "Abbey", klass: k1)
s2 = Student.create(name: "Brooke", klass: k1)
s3 = Student.create(name: "Courtenay", klass: k2)
t1 = Teacher.create(name: "Miss Jane", klass: k1)
t2 = Teacher.create(name: "Miss Martha", klass: k2)

Klass.first.name
# "Room 1"
Klass.first.teacher.name
# "Miss Jane"
Klass.first.students.first.name
# "Abby"