20130611021300_create_users.rb

class CreateUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.string :name
      t.timestamps
    end
  end
end

20130611021313_create_courses.rb

class CreateCourses < ActiveRecord::Migration
  def change
    create_table :courses do |t|
      t.string :name
      t.timestamps
    end
  end
end

20130611021353_create_course_teachers.rb

class CreateCourseTeachers < ActiveRecord::Migration
  def change
    create_table :course_teachers do |t|
      t.integer :course_id
      t.integer :teacher_id
      t.timestamps
    end
  end
end

20130611021359_create_course_students.rb

class CreateCourseStudents < ActiveRecord::Migration
  def change
    create_table :course_students do |t|
      t.integer :course_id
      t.integer :student_id
      t.timestamps
    end
  end
end

course.rb

class Course < ActiveRecord::Base
  attr_accessible :name
  has_many :course_students
  has_many :students, :class_name => "User", :through => :course_students
  has_many :course_teachers
  has_many :teachers, :class_name => "User", :through => :course_teachers
end

course_student.rb

class CourseStudent < ActiveRecord::Base
  belongs_to :course
  belongs_to :student, :class_name => "User"
end

course_teacher.rb

class CourseTeacher < ActiveRecord::Base
  belongs_to :course
  belongs_to :teacher, :class_name => "User"
end

user.rb

class User < ActiveRecord::Base
  attr_accessible :name
end